"The full answer's a little complicated but the shorter answer is, it's usually Evil but it's not always Evil. —Uh, and it's not just Asmodean propaganda, lots of places have propaganda about it. I think the Asmodeans might push it even harder, since they really don't want people to think they can just get out of living in Cheliax that way and be fine?
What the Good churches back home say is that — if a man abandons his whole family and they starve, that's Evil, and it doesn't really matter whether he's abandoning them to go adventuring in the River Kingdoms or whether he's abandoning them to be dead, and that lots of people have some sort of responsibility like that, that it wouldn't be okay to completely abandon, especially not without telling anyone beforehand. And that most people who think killing themself will make things better for other people are wrong, and that if you have a reason to think you're different you can talk to a priest and they'll set you straight — that's not exactly how they phrase it, they make it sound like they won't go in with their minds made up. And that even if it wouldn't be Evil for you to commit suicide, you might still have been Evil to start with, and in that case you're better off staying alive and trying to make up for what you've done.
But they don't say it's always Evil no matter what. There's a story in the Acts of Iomedae, the Eighth Act, where there was this undead guy called the Black Prince, and he was definitely Evil, and rather than just killing him right away, she stays up all night talking to him about all the Evil things he'd done. And somehow she convinced him that it was wrong to do those things, and even though he was undead he still felt really guilty about them, and he wanted to repent. Only because he was undead he couldn't just stop, the way a human could — that part's kind of confusing but the Iomedaeans get really mad if people try to leave it out, they're so picky about how people talk about that Act — and so he killed himself so that he could never hurt anyone again. And even though he'd been Evil beforehand he still went to Axis, because his suicide had actually been Good, the same way it would've been Good for an adventurer to kill him.
And — if Cheliax is going to Maledict you, and the only way to stop them is by killing yourself first, then — you're not abandoning anyone who needs you, because you'd be dead anyways. You've got a good reason to think it'll make things better, because they won't be able to get information out of you, and they won't be able to send you to Hell and try to force you into being Asmodeus's slave. But even if Pharasma decided that she'd damn anyone who killed themself, I'd still rather kill myself than get captured by Cheliax, because I don't want them reading my alive friends' names and faces and plans that I know of out of my head, and I don't want my soul to be doing anything useful for Asmodeus, even just being a paving stone, and I'm sure the Abyss'd suck too but I'd take it over that."