What Do You Make Of the Ending? [Spoilers]

I think that Jack Delroy made a deal with the devil/demon for him and his show to be succesful and I guess because of that his wife got cancer.

What do you mean? All the trippy stuff that happened after Jack Delroy ran off stage? Here's my take: In short, he was part of 'The Grove', which that satanic cult also had something to do with. There he took part in a ritual, where he made a pact with the Devil and sold his wife's soul; all to become the number one talk show host. He loved his wife, obviously, and didn't think it would work, but he was desperate, and in the end it did, in fact, work. I'd assume he did become number one that night, beating Johnny Carson's ratings... but only for one night. That's how making deals with the Devil works, I guess. Be careful what you wish for.
I dunno, I've just finished watching it myself and I'm still kinda processing it. I'll have to rewatch it again in a few days. But overall, I liked it.

I also read a theory about his wife being a vengeful spirit and that it's her who's after him. But again, I'll have to rewatch it. I do remember her popping up throughout the film, every now and then.

I don't care. It was hard to watch from the beginning but I stuck around because it was playing around with some familiar elements like the Amazing Randi and Anton Lavey and so on. But I never watched Johnny Carson or any of those other shows, so this was mostly just dull to me.

Not a patch on Ghostwatch and generally not worth the time

Jack sold his soul to the devil. His wife died as part of the deal, so once he made that payment, he would get what he wanted: to be Nr 1, and better than Johnny Carson. When that show aired, best believe no one was talking about Johnny Carson, but about Jack Delray. So he got his wish.
But the payment was too high. Not only did he lose his wife, but his soul now belongs to the devil. He is now in some sort of purgatory. He is trapped in a endless circle of his show, but a twisted version of it and memories of his wife and his Grove ceremonies. He knows it's not real, but can't wake up. He tries to use Carmichael's line: Dreamer, wake up. Nothing works. He is now back in the studio, with dead bodies all around him, and the police siren getting closer. He will either be sent to prison, or a mental institution. Either way, his life is ruined by his ambition and greed.
P.s. Minnie is not the only one who he sacrificed. He also got June to do the summoning, even if she didn't want to and told him how dangerous it was. Gus also warned him and didn't want to take part in it. Now they are both dead because of Jack.

Do you mean the very end, where he has killed Lilly?
Did he kill everyone in the studio I wasnt sure, im guessing like everyone he made a deal with the devil or some kind of sacrifice at the Grove but wasnt sure about the mind bending trippy part then he wakes to be holding Lilly having stabbed her?