Sorry, I didn't spoil anything because we're in a thread comparing two movies, so I'd hope everyone posting in it has seen the movie, and those reading it have as well so as to gain something out of it other than shallow platitudes and punctuation.
Folks, he knows Clark has a Mom.
He specifically thinks Clark is an alien who is an outsider.
Clark telling him to save someone as his last words means something.
You could take out Lois, but then Bats could think Clark is fucking with him and kill him anyways. Lois, someone he doesn't know who is decidedly human telling him this, tells Bats that he has become who he hates, Joe Chill. We could talk about the acting in the scene, which is a given since its Ben Fucking Affleck, but the point is made.
After Bats realises what he's become, a Jeremy Irons monologue made flesh, he immediately wants to not be that and do whatever he can to help. Him saving Martha is his personal redemption for not being a fighter that night in the alley.
To reduce this to Bats and him have moms is severely understating what was really going on. Again, its a comic book movie about two of the world's finest good guys punching each other, why they do it in the first place is always hard to swallow.
And Lex's plan was to make earth ready for Darkseid. He thought Clark would kill bats(his other potential threat), but be weak enough to get killed by doomsday. He didn't plan on the trio uniting.