Ooh, interesting question. I like to play around with the idea of life and death a lot in my stories. As for remembering murder, it depends on the story and the character's background. If it matters to the character then I can immediately recall it. If not, then no.
For my biggest projects, some of my lead characters are actual (original) gods. Killing is not a big deal, ultimately, as they are creators, and cannot be held to the same standards as people. They are creation itself and can do with existence as they please.
Others, they've grown up in an environment that makes them understand you can't hesitate to kill someone. You never really question if it's wrong or right, because no one has ever questioned it in that environment before, and when someone is attacking you it's simply self-defense. To leave them alive afterward would be foolish in the setting, though no unheard of. Life is not sacred, especially in a universe that fully acknowledges, and knows that reincarnation, and more, is a thing. Anyone can begin again. I don't remember any of their first kills, really, and neither do they.
In some of my other stories, however, my protagonists, and other characters, apply much more value to life regardless of spiritual beliefs, being reborn, reincarnation, and so on. One character in particular has a situation in which they literally have no choice but to kill--the control is taken away from their body on a regular basis. From the first death, all the way to the hundredth death, they remember every single one, and it takes a massive toll on their mental health. While, yes, throughout the 2/3 of the story, they seem badass and extremely capable, you see in the end that they'll never be that way again. They're left completely broken, and disgusted by what they had to do, no matter how much others try to justify it or explain it. They can hardly get out of bed most days, or eat, and have to be monitored by others to ensure they do. Super depressing