So, the character is an anti hero, he loses all his morals, and becomes the very thing he fights against.
How can I make the downfall of this character more emotionally moving/impactful without making it seem contrived or cliche? Are there any things to watch out for when doing this?
Usually people find things moving when they identify with them. So your best bet is to start with an identifiable character, and then show that person making choices that, while they might be wrong, the audience can imagine themselves making.
Consider the show "Breaking Bad." The main character is originally a likeable person in a common profession. His initial choice, to start manufacturing drugs, is understandable, given his circumstances --a limited amount of time to live, a family to take care of.
As the choices become less justifiable and more immoral, the audience, once identified with the character, continues to think "I'm just a few wrong decisions away from being in his shoes." That makes his downfall moving.
The best method would be to make the downfall fairly slow. Make sure it is quite detailed, and let the reader truly understand the downfall. I suggest a good deal of inner dialogue. As for what you should avoid, try and keep the anti hero in character. For instance, if they are rational and like to think things through, have them reason out why they are doing this. So, to summarize, don't rush it or alter the character's personality. One other technique, though it is used often, is to show the emotions of those the character knows.
The plotline you're describing (good guy goes bad) is, in itself, cliche, BUT that doesn't make it intricately a bad thing.
The key to doing this well is to make it believable. To make the reader connect with the character, even as he's turning evil, because the circumstances allow for no other reasonable option. Anything short of that will most likely fail to convey the right amount of angst.
One of the best falls of this is the Character arch of Demona, who we first meet as the Hero Goliath's love interest, and after a 1,000 year time skip (following Goliath's belief she was killed) we meet her again and find she's very much changed from the person Goliath remembered. The four part episode titled "City of Stone" reveals that Demona had an unwitting hand in the clan's massacre and initially hid from Goliath, leading to the 1,000 year slumber of him and the remaining survivors occurring before Demona had been given a chance to properly reunite (i.e. Not reveal herself in an incriminating fashion)... when she finally sees the sleeping hero, she utters the line "What have I... What have they [The Humans] done to you?!"
The line would be critical to Demona's entire character and would resurface in similar motivated characters. Demona admits in this one line that she knows exactly who is to blame for the slaughter of the Gargoyles and the curse brought on the Survivors... but she cannot admit her own culpability to herself... she had made the deals to save her people from human brutality... but it only resulted in their extinction. The weight bears down on her and she comes to show hatred to humans... without knowing why the betrayal happened at all... and even those humans who were never there in the first place are shown hostility by her. In her old age, nearly dying and on the run, she wishes for restoration of her lost youth to protect her people... at the same time a young king wishes for the means to defeat his enemies... this locks the two a human and a Gargoyle together, unable to die but for dealing a killing blow to the other. And for 1,000 years, she believed humans betrayed her and her hatred for them festerd so that by the time she and her beloved are reunited, she had 1,000 years of being the last of her kind in the world of her enemies... while he had a good night's sleep.
As the episode concludes, the events of the modern part of the City of Stone episodes are revealed to be directly because of her past as a woman who Goliath once knew as the "Angel of the Night" becoming the someone named "Demona", the one who fights like a demon. Each reasoning given for her successively worse actions is reduced to the next, until she is forced to admit that which she never could for the past 1,000 years. Who is to blame for all of this? "I am."
Demona had opportunities to stop what was to happen. From the earliest moment, she was given a chance to admit to what was about to happen to Goliath, who could bring attention to the problem and resolve the conflict... she seriously considers it... but decides to not let him know. But for this one action, Demona would never have happened... she would remain the Angel of the Night. But she felt that she could not hurt Goliath by burdening him with her collusion... and by the time it was all said and done... The pain she caused was cemented in the stone of time, marking her forever the villain and forever alone.
As others have said, you need to make the bad decisions believable by the reader.
The easiest way to do that is to make the hero struggle with terrible consequences for doing the right thing.
An example of this is given in a preview (of some series I am not watching, and cannot recall the name) in which, in a 30 second clip, a law enforcement agent is given the choice between her kidnapped child being killed, or ratting out her team. Apparently the bad guys have her kid, she is convinced they will kill her, and she panics as their countdown progresses, then screams the names of her team into the phone. They all end up dead.
The price of righteousness can be too horrifically high for just about any of us, so high that even if we were willing to sacrifice ourselves, we can't bear to sacrifice somebody we love (as a parent, sibling, child or mate).
This is why most "good bad cops" we see on TV (Good guys that occasionally break the law, like conducting an illegal search, or Jack Bauer beating a confession out of somebody, or using tricks to keep a bad guy more than the allowed 24 hours) are still seen as good cops, not criminals. Writers are usually careful to show the audience that the bad guys are really bad, so the actions of the good bad cops feels justified to the audience: If they don't break the law, something awful could happen to harm innocent people, so the good bad cop seems like they are still acting in the best interest of the public. (Until they are completely wrong about the guilt of someone, and therefore harm somebody innocent by sending them to jail or getting them killed).
You follow up these "unavoidable" transgressions with further voluntary transgressions to cover up those transgressions. Back to the good bad cop: She doesn't want to go to prison or be disgraced -- So if somebody suspects something funny in a case she secretly torpedoed, she exploits her position or their trust to interfere with their investigation, so they can't find the truth.
Eventually these crimes escalate, or backfire, and the cost escalates to voluntarily doing something evil, like intentionally killing somebody to protect themselves and avoid punishment. This weighs on their soul to the point the only escape is numbness, the only rule is their own survival at any cost. That doesn't have to come with greed, but maybe it does.
They have been turned, their morals are ruined. But hopefully you have brought the audience along seeing that for every choice, the consequences of doing the right thing was terrible, and the "hero" wasn't strong enough to take it. The first such choice must have such horrific consequences it breaks him.
But after that one breaks him, you can progress down the scale from "doing right is an impossible choice" in steps, down to "doing wrong is the convenient choice". Once his morality is broken, it only gets more broken.
