It isn't so much the act, but the nature of the act in this instance, that's (possibly) horrifying. Assuming they had kids together, who could possibly condemn someone with whom one once had a loving relationship, to a few seconds of mind numbing terror before eventual death? Killing someone you don't know in similar circumstances or killing someone 'humanely' (daft choice of word) who you do know is bad enough, but this has the worse of both worlds.
One of my old COs was convicted of killing his wife when he fell for a foreign translator. Weird, if not ageless, motivation.