Well, in the Abrahamic religions suicide is a sin, so no surprise they're against that. Also iirc being sad and depressed is a sin as well.
But I'm not a Christian and I'm against assisted suicide in cases when the person is fully physically healthy apart from being fucked in the heada.
I think it's bad because in modern society we have false views on mental illness and the way we're treating them is wrong. Also, the fact that person is fucked in the heada and depressed right now doesn't mean it will always be that way, they could changed their mind in a week, but we won't ever know that.