Revealing mistake: What are the odds for twins to have birth marks on the exact same places? When Isabel commits suicide and lands in to the pool you can see she has the exact same birth marks as Angela to the right above her lips and on her left neck. (00:13:30)
Revealing mistake: When it's raining a lot and John is taking a cough suppressant, he sees some kind of bird fly by and looks up. There's a shot of his face from above and the rain is falling around him but not on his face. (00:26:40)
Revealing mistake: When Chas is being thrown up and down between the ceiling and the floor, if you look closely as Chas is falling from the ceiling to the floor and goes up again, when he goes up it's just a rewind of him falling down. (01:28:35)
Answer: Sin for a good reason is still sin, and as Gabriel says earlier, you can't buy your way into Heaven. Real Catholic dogma, however, doesn't hold the mentally ill as condemned for committing suicide.
Greg Dwyer
Except Isabel wasn't mentally ill. She saw angels and demons just like Constantine did. It was her parents who believed she was mentally ill.
While suicide is a mortal sin, it's shown later (as in major plot point) that sacrificing yourself to save the world is a redeeming act.
Yes, but Constantine also said "My parents were normal. They did what most parents would do. They made it worse. You think you're crazy long enough, you find a way out" which could relate to Isabel losing her sanity in a way as well because of her family and how they saw her. The whole Isabel's sacrifice is added by the novelization but the movie is ambiguous about the suicide.
mp1920
Except Isabel wasn't mentally ill. She saw half breeds just like John did.
Sacrificing yourself for others isn't a sin.