Deliberate mistake: When Constantine comes out of the library after speaking with Gabriel, he stands beside Angela and none of them cast any noticeable shadows on the door and walls behind them. When it cuts to a shot above them, shadows are more being cast behind them. This is probably because of different light settings on the set. (00:23:35)
Deliberate mistake: When Constantine and Angela are driving while talking to Beeman, as Beeman says "But that doesn't mean that we don't have faith in you", the lights in the car suddenly go off. Revealed by the DVD commentary, it is the cinematographer's lights going off by mistake. (00:57:50)
Deliberate mistake: When Constantine and Angel are walking to her car the power shuts down turning off all the lights in the street. Then it cuts to inside her car where you can see the lights inside the car is on. When it cuts to outside the car the lights are off. (00:37:35)
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.
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Except Isabel wasn't mentally ill. She saw half breeds just like John did.
Sacrificing yourself for others isn't a sin.