Ellis is content to watch. He is a demon who is sustained on small evils... sins...
You have the moral bastion that Taine is trying to be, all about consent and not using/abusing the body he's been entrusted with. The ethis of say, sleeping with someone younger than him, in a borrowed body without permission... That's Ellis' delight. He wants the good man to fall, to give in to the temptation, and boy o boy is Tyler one big old package of temptation.
Taine had no real choice but to tell Tyler, it was the right thing to do. Present the truth, even if it makes you look insane, and let Tyler choose what he wants to do. Tyler surprises me repeatedly in this first book, he's not afraid of the situation, and likes Taine for who he is, not what he is.