If the man in question knew the woman was married, he is doing something wrong. But we don't know that. And we may also not know that he might have tried to help her get out of the abusive relationship. Feelings happen, and it's easy to be blinded by them. It's not justification, but say they met and really liked each other, and while he was spending time talking with her about the fact that she should get out of the relationship with her husband, they may have ended up in the sack. It's sadly a mistake that's far too easy to make.
What I think I mean I'm thinking is that this could've been a well-meaning man that found a woman he really liked, found out she was married, tried to figure out the situation, maybe help her if she felt unable or unwilling to leave what might've been a bad relationship. It may then have been a slip-up by an otherwise well-intentioned guy. Whatever that would mean to a moral compass is very individual, I think. Of course, he may have been a self-centered ego-maniac that found a new girl to fuck, and this girl was attracted to that in him. Who knows.
All I mean to say is that out of everyone involved, the guy deciding to murder his wife's lover is the one doing something completely irrevocable and bereaving someone of their life. It is so out of proportion to anything that I hardly think "that's just how reality works" can qualify it, much in the same way that reality also works in the way of having adults molest kids, and it doesn't change how wrong it is.
Yes, there are fucked up people out there that will decide to murder of someone that might be the least to blame in the situation, rather than sit down with their wife that cheated on them, and figure out why it happened, and either work together on improving the relationship, or deciding to break up. It seems a man's way of justifying his insecurities, going after the one your girl cheated with, and it doesn't keep your girl from doing it again.