Well if she is super pissed at him, and it's obvious she isn't really spreading the word of how fantastic he is, he might want to off her to shut her up because she is spreading dissidence. If he is desparate to keep a good name for himself, and keep his position of power, he might be moved to kill his wife with whome he is fighting with. Remember though, I was talking about their relationship at the very beginning of the series. It's been a while since that book club meeting by the time he tries to drown her.

And we don't know that that wasn't just a ruse to make Jack sympathetic to her.