Yes, we are all murderers at heart. We are all killers at heart, for food.
Some evolutionary scientists believe we would not have evolved brains without eating meat on a regular basis; regardless we are particularly well suited to long distance running which is probably how our earliest ancestors in Africa hunted: Running all frikkin' day, chasing animals. Because with our sweat glands and light body hair, we can do that, and they cannot: They die of heat stroke in a matter of hours if it is hot; we do not. Our sweat cools us off; a deer can only pant. (Like a dog, dozens of young dogs in the USA die of heat stroke every year because their human owners run with them for five miles in warm humid weather, and their obedient dog has a stroke and dies).
It is in our nature to kill large animals for food, and it is also in our nature to fight and kill our rivals. There are no "base" instincts and "higher" instincts. There are instincts, and they are all equally valuable, and equally weaknesses or strengths.
In that sense, the rule "an eye for an eye, a hand for a hand, a life for a life" is just restating a part of our basic instinct, but even such a simple law, while defining fair retribution, falls short by failing to define the circumstances in which it should be invoked: Specifically, a selfishly taken "eye, hand, or life". Most agree if a man takes a life in self-defense, that might be fair even though his life was not taken. But perhaps not fair if he had an easy alternative to avoid attack, or was taunting his attacker, or was raping his attacker's daughter when he was attacked, etc. So our sense of fairness is not super simple, and so difficult to define precisely it almost always falls short -- Which is unfair! Laws should never be taken as gospel in determining whether a treatment of a person is "Fair." If I drive at ninety miles an hour on city streets I am breaking the law, if I run red lights and refuse to stop when chased by cops I am breaking the law, but most juries will exonerate me if my brother is bleeding to death in the front seat and I am trying to get to the Emergency Room. That is "Fair" because of the exonerating circumstances.
There is a biological reason the average male is 25% larger than the average female, and more prone to muscularity, height and speed: The male body is designed to FIGHT, particularly each other for the right to mate. The same is true in the super-majority of other species; males are specially adapted to fight other males for mates, and females are not. Throughout the animal kingdom, the majority of males have no offspring at all, while the majority of females all have offspring. This is due to the minimal amount of time and energy it takes for a male to father a child, and the maximal amount of time and energy it takes for a female to bear that same child; in humans that can literally be a few minutes versus nine months of increased calorie requirement and about five months of increasing disability, a 200,000 to 1 disparity, not to mention two years of continuing care for the infant!
This disparity in efforts for reproduction has led evolution to produce not just physical differences, but psychological differences that are gender-linked, and those in turn create cultural differences across the world in how women are treated, protected / enslaved, pursued, and mate (whether by choice or coercion).
The evidence for this is on the ground, the vast majority of murderers, worldwide, are men. The vast majority of rapists, worldwide, are men. The vast majority of sexual slaves, worldwide, are women, owned by men. (However you wish to define it, I define it as any woman repeatedly forced by the same individual to have sex she doesn't want to have.)
There are cross-overs, both in sexual preference and murderering, rape and slavery, but we should not expect anything in biology to wind up 100% iron-clad, even physical gender can be mixed, it should not surprise us if psychological attributes that are highly correlated to one gender are sometimes found in the opposite gender.
We are murderers at heart. There are few emotions that did not serve us at some time or another in our rise to humans. Anger and rage and violence serve a purpose still. Murder worked positively to stop predation on people by other humans long before we had any laws. Murder of rivals, kidnapping and rape of women, violent competition for women and resources (including all out war) all worked negatively in pursuit of the only thing that ultimately drives evolution, reproduction. A man's competition cannot threaten him or out-reproduce him if the competition is dead. (Of course then, the winner's male offspring may inherit his physicality and psychology and have to compete against each other...)
Violence and killing come naturally to women, as well, defending themselves and their own offspring, hunting for food (they need meat to fuel their brains as much as men do), and so on. I am not suggesting here that women are incapable of violence, but statistically speaking, worldwide, they just do not murder as many rivals as men do, and nearly never forcibly rape anybody.
It is not in our nature to respect "the rule of law," otherwise we'd never overthrow a despotic king. More generally, how would we ever know to make a law against something, what in our nature makes us say "that should be against the law?"
What humans (and chimps, dolphins, elephants, corvids and perhaps other species) are born with is a not-too-simple but not-too-sophisticated sense of FAIRNESS. It is that sense upon which "law" resides, and it is that sense that tells us when a law is unfair, or something is happening that is unfair and we should make a law against it. The words "justice" and "just" and "unjust" all derive from the Latin "Justus" which meant "Righteous, Equitable". Equal. The law is a poor and often vague implementation of the fairness most of us have an instinct to demand, and take when the law fails us. It is why are Lady Justice holds a scale, which is fundamentally the metaphor of determining whether things are unequal or unfair.
So yes, we readers understand when a character kills somebody else, whether for their own selfish reasons (the killer deserves punishment!) or altruistic reasons (the killed deserved punishment, and equivalently this prevents the killed from doing additional future harm).
Both scenarios are in our blood. The first is there because "selfish" is quite literally what we are doing to reproduce our self! The second is there out of our inherent sense of fairness: If we believe somebody is doing great harm and will continue to do so, especially if they have the power, resources or contacts to circumvent punishment by the law, then we feel it is still "fair" if they receive punishment, and we don't really care what the law may be: Fairness comes above law.
Rage may seem destructive, but it serves an evolutionary purpose, in some circumstances completely ignoring consequences and trying to commit murder really is the only way to have a chance at spreading genes or protecting the genes you have already spread (children). It may fail, but it may succeed where surrender, submission or refusing to risk death or refusing to murder is guaranteed to fail. Similar arguments apply to other emotions, like jealousy, hatred, vindictiveness, etc. Due to biological imprecision (and not growing up in anything like natural setting or social environments) these are frequently misused, but they have perfectly valid evolutionary justification for existing, they have contributed heavily to our development and intelligence.
If you want to write about the human condition, it is possible but difficult to do while skipping the most important part of it. There will always exist some psychopaths in our midst, perhaps 1/4 to 1/2 percent of people are born with flawed feedback nerves in their brain that prevent them from feeling guilt or empathy. That is also part of our nature, having to deal with such people, and they are the enemy of mankind driving most of our law since the modern human brain emerged in force about 50,000 years ago (and the first woman with it may have existed 100,000 years before that).
Killing people to prevent their future action is a part of us, and is one of the easiest things we can identify with. It is only "murder" when done for selfish gain or pleasure; we tend to disagree with the label "murderer" when we speak of soldiers killing the enemy, or a girl being raped shooting and killing her attacker. We don't even think the soldier or girl should suffer regrets, and if they do, we feel sorry for them being traumatized by having killed.
On the other hand we are appalled when a murderer feels no regret, then we revile them and think they're defective ... and deserve to be killed to prevent their future actions! Because gain (in property, money or pleasure) by murder is fundamentally unfair no matter what the "law" says, and if the law does not agree, then it is the law that is flawed, not the instinct.
Why do bad characters sell, and does this ultimately serve base instincts or higher instincts?
Bad characters sell because the vast majority of us possess the instinct for fairness and we want to see the bad people get their fair due, that is entertainment for us. This serves instinct in general, I reject the division of "base" and "higher" because I think both are critical for a healthy mental life, and understanding what drives both helps us navigate life being fair to others: Which sometimes necessitates punishing them.
I believe a valid distinction that can serve is "selfish" and "altruistic" instincts; but killing is a tool and not automatically one or the other: If Joe killed a man in order to protect a woman, he is altruistic. If Joe secretly killed a man loved by a woman to get that man out of the way, Joe is selfish.
So it depends on why your character killed a politician whether the reader will consider it a murder or a justifiable killing.