Yes, technically suicide, or assisted dying, is a sin. It's not so much about not angering god, but more showing trust in him. Christians are not really controlled by God, but strive to follow his teachings - and some things are sinful because it is against that. However, as someone mentioned up thread, everyone is sinful from birth - no one is perfect and everyone falls short, but that's what redemption and forgiveness is all about.
And if you believe in a deity, any deity or higher way - and you really, truly believe it - then why would you not act when you think others are putting themselves in harm's way (as you see it). Why would you not speak up to say you think it's not OK? I am the first to say a lot of religion is about controlling other people for nefarious gain but faith - that's different, and seeks to protect. People are free to disagree with them but I get why they do it.
The view of an unproven figure is more important than an individuals' own feelings / thoughts / views about themselves. Fascinating.
You can make it about that I guess but actually it's entirely internally consistent. If you believe god is real, and his message is real, then yeah, his way must be perfect and therefore of course it trumps individual people's wants. But free will is a thing, as is a compassionate and loving god so it's not all fire and brimstone as people think.