Factual error: When Angela is watching the security video on her laptop of her sister committing suicide, you can see she's playing it using Quicktime. She rewinds a couple of seconds by clicking on the first button of the video player. Actually, when you click this button, it takes you to the beginning of the video. The second button would have rewound it a short time. (00:25:20)
Factual error: Rachel Weisz fires approximately 29 shots from her semi-automatic handgun without reloading - vastly more shots than her gun can hold.
Factual error: When Angela talks to a priest about giving Isabel a Catholic funeral, the priest says that it's impossible because Isabel committed suicide. Even though the church does consider suicide a mortal sin, this rule doesn't apply to people who have a history of mental illness.
Factual error: After John traveled to hell using Duck the cat, he and Angela are going to eat something and John tells about his suicide attempt. In this flashback, you see them using a defibrillator to shock him back to life. When he is shocked, people are touching his body, but they don't get shocked.
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.