How do you write a character with goes from A to Z and back to A realistically? Let's say the characters is extremely in love, then wants to murder the person she was in love with and then go back to being completely in love. How do you do that realistically without resorting to cheap tricks like amnesia, magic and other similar things? I don't think this can be done realistically at all.
For Z to have value, then there is no going back to A. The path is instead
A -> Z -> B
where B is similar to A but includes the added experience from Z.
In your example, the character is madly in love, she then finds out that she has been deceived, hence the murderous anger. The love interest then shows a deep repentance, guiding MC to feel the love she felt initially. This new stage is however one in which she has more experience, the love is more 'mature', less blind, and hardened by the now confirmed trust.
On the other hand, a true path like
A -> Z -> A
tells the reader that Z was of no importance and all the time they invested in the story was merely a blip that could be reset with no consequences. This may be negatively received.
I think the question is wrong to put extreme love and murderous intent at opposite ends of a spectrum and to suggest that there is a substantial distance between them. I think in fact that they are both expressions of intense emotional involvement with another, and you only have to read the newspapers occasionally to see how closely related murderer and victim generally are.
But therein lies a solution to OP's problem. I'll explain:
P and Q are lovers, totally devoted to each other and, both inwardly and outwardly the epitome of extreme love. P sleeps with R (not entirely sure why, that's something for the writer to figure out), Q finds out and flies into a murderous rage (hasn't got to actual murder yet so maybe it's really only a not-quite-murderous rage). This is a previously-undiscovered flaw in Q's character (another hook to hang part of the plot from). But P and Q haven't had to travel far to get from where they started to where they are now.
The journey back is likely to be a bit harder, and to take a bit longer. Somehow P is going to have to acquire Q's forgiveness (or Q is going to have to accept that P's intense love does not entail fidelity), and Q is going to have to learn how not to fly into a murderous rage. I don't think these emotional journeys for P and Q require any of the cheap tricks you are (rightly) keen to avoid, I think they are well within the experiences of many people.
Let's call our girl Alice, and her love interest is the naive Jack. We totally frame Jack. In her eyes, Jack is guilty as hell, over the top, she trusted Jack with her life and Jack destroyed it. Hard. It is because of Jack she lost her fortune, lost her job, even lost her parents in an accident, whatever.
But, Jack did not really do any of that. He is intentionally framed, by the real villain, Jill, a sociopath evil genius Alice was casually hooking up with, but ultimately broke up with, not because Jill was evil, but because Alice was in love with Jack.
So out of spite, Jill sets about methodically destroying Alice's life, and making sure all the blame falls on Jack. And it works.
Alice hates Jack. By mid-story Jack gets prosecuted, wrongly convicted, and Alice is happy about that. But then Jill resurfaces. She comforts Alice. And Alice accepts, they hook up, Alice and Jill are back together, and all looks like roses to Alice.
But then Jill slips up. She knows something she cannot have known about what Jack did to Alice. (We plant this secret incident early, of course). And Alice grows suspicious, she starts secretly pulling this thread, and finds Jill's fingerprints everywhere.
Alice realizes that Jill framed Jack. she was gaslighted. She secretly goes and talks to Jack, who is still in love with her. The more she looks, the more she realizes that every evil thing she thought Jack did behind her back was all Jill. Jack was just a naive patsy, conned at every turn by Jill's minions.
Alice is ashamed, but Jack is a saint, he forgives Alice. With all guilt erased, they entrap Jill. She is found out for the sociopath fraud that she is, and in the end, Jack and Alice have rebooted their love relationship.