Orcat
I don't know if I can help much with the insight thing, but I've killed more than my fair share of people. I didn't have a choice with all but two of them, and with the exception of those two I wouldn't have made any different decisions. But those people probably had families, or people that loved them, and there's nothing I'm ever going to be able to do to make things right with them. If I tried, it'd just do more damage than good.
Maybe we can look back at the shitty things we've done and say 'this is how I'll make sure I never do something like this again' and maybe we can't. But either way, it's something we've got to live with one way or another, and that includes living with the knowledge that we can't actually make it right. So I guess your options are learning to live with it, or letting yourself drown in it.