John Constantine: When I was a kid, I could see things. Things humans aren't supposed to see. Things you shouldn't have to see. My parents were normal. They did what most people would do. They made it worse. You think you're crazy long enough, you find a way out.
Angela Dodson: You tried to kill yourself.
John Constantine: I didn't "try" anything.
Angela Dodson: Well, this has been real educational... But I don't believe in the devil.
John Constantine: You should. He believes in you.
John Constantine: So when a half-breed breaks the rules, I deport their sorry ass straight back to hell. I don't get them all but...I'm hoping to get enough to insure my retirement.
Angela Dodson: I don't understand.
John Constantine: I'm a suicide, Angela. When I die the rules say I got just one place to go.
Angela Dodson: You're trying to buy your way into heaven.
John Constantine: What would you do if you were sentenced to a prison where half the inmates were put there by you?
Angela Dodson: I guess God has a plan for all of us.
John Constantine: God's a kid with an ant farm, lady. He's not planning anything.
John Constantine: I did tell you to move the car.
Chaz: Yeah, well if you'd have told me that there was going to be a 300 pound mirror with a pissed off demon inside it falling from a window, I would've moved it further, John.
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.