For almost everyone who says that on the internet, they are nat saying it with almost any sense of sincerity. They don't want the person to actually kill themselves, they just say it.

They say it because in their circles, that's a perfectly acceptable thing to say, and no one would interpret it as anything more than an expression of disapproval. (think 4chan, many subreddits, etc.).

They then forget that outside of their circles, people aren't necessarily privy to such culture, and say it anyway. It probably doesn't even cross their mind that they're saying something hurtful.