The thing you mentioned shows that the characters in this manga are really human. Nobody can endure such events without starting to question their own beliefs.
I also think that the fact that Usobuki tried his tricks on Tada on his own means that he was hoping Tada would not fall for those. He is the first person who picked up his interest.
Usobuki plays God and looks at the world as a big game. The game of life and death. But don't you think that the idea of a god falling would be like a big finale of a show for him? He wants to die when the "game" reaches its climax, so whether Tada loses his sense of justice and kills him or he decides not to, Usobuki would win. It will always end with "how foolish these humans are"
I have a love/hate relationship with this manga but I'm going to drop this if Tada "succumbs to the darkness" and dies. I thought Tada will never succumb to Usobuki and give up his sense of justice but things are not looking up right now, especially since he silently declared that he would kill Usobuki. Ah but I don't want Usobuki to die either, as much of a bastard that he is. Can't I have both???
Interestingly enough though, is Tada the only one Usobuki tried to kill on his own volition? First the pancake, then the knife, and then somehow the dresser.