He is a douche. He never liked Gatsby, and he had to finish the job after breaking Gatsby down. He has to make sure that that Daisy can never again be with her secret lover. Gatsby is too dangerous when he is paired with Meyer Wolfsheim, so Tom must get rid of one of them and Gatsby is easier to get rid of.
Toward the end of chapter 7 Tom and Daisy are seated at the kitchen table with an "unmistakable air of natural intimacy ... Anybody would have said that they were conspiring together." Fitzgerald is a deliberate author - he would not have used this word if he did not want to imply that in fact Tom was aware of what would happen.
Tom is innocent because he didn't know Wilson was going to kill Gatsby and because he didn't necessarily say it wasn't Gatsby who was driving it's Wilson's fault for assuming Gatsby was driving and killing Gatsby without even asking him if he knew Myrtle. It was all Wilson's fault yes.