Milly, my ex did kill someone. I went on to marry him and even though we're not together anymore, we are still friends and I trust him totally.
Want to know the difference between him and your guy though? He told me on our first date, told me exactly what happened, it was an accident (not one of these punching someone and them hitting their head type accidents though). I could sympathise, which people reading this might be shocked by but if I told the full story I think most would understand. He told me on that first date because he was ashamed, not a day went by when he didn't think about it, when he didn't re-live it and torture himself with it. He thought I had every right to know what he had done before I got involved with him, he certainly didn't "forget" about it. He never once, not one single time, joked about it or talk about it in a light hearted way, hell it almost destroyed him, and this was more than 10 years after it had happened.
My ex was not a violent man, he never hurt me, we argued at times sure, but I was never scared of him or worried that he might hurt someone else, he was a good man, and that's why he found it so hard to live with what he did,
You, I have to say, sound almost impressed by his actions, you come on here defending what he did when if he was truly remorseful he wouldn't even defend it himself. If he can "forget" what he did and see it as not even worth mentioning, then he IS NOT a good person. If you want to blind yourself to that fact, go right ahead, but it's only yourself you are cheating.