Pride, mostly. After years of planning he's formulated the "perfect" plan and doesn't have any doubts. He wants this to be a show, not stealth-kill. He wants to humiliate Superman, who dared play god in his turf. So, yeah, I'm going with vanity and the lust for revenge. He wants to slay the beast, not simply kill it... That, or they just wanted a fight scene in the movie...
Cause he's smart yo! They never explain it, but eh, the explaination is probably "he's a genius", which I don't have a problem with.
What bothered me is how Superman also suddenly knew Batman's identity, and Bats didn't even flinch there. I mean, this guy you wanted to kill knows your secret and you didn't even acknowledge it? I get that they had "bigger" problems, but it seemed weird to me how little Batman cared that this invulnarable "murderer" knew his secret, and possibly for a while now.
Which wasn't explained either, how Supes guys from not knowing who Bruce Wayne is, to uncovering his alias. Eh, I guess when the mask was half-broken he recognized him, it's not too far fetched.