Why wouldn't it be? Why's it any worse than calling someone, say, a wanking bucket of festering sputum? It's not like someone's going to read "kill yourself" and think "oh my, I'd never considered that!" then suddenly give it a try. It's just another insult.
Oh, and kill yourself.
Trust me, I know some very depressed people. The difference between that and the wanking bucket thing is that the wanking bucket insult is funny.
For instance I might say that ViolenceIsGolden is a pickled twat biscuit in a lake of stinking horse jizz. Or I might straight up tell him to kill himself. You never know, he might act on it. Or think, "LOL, horse jizz" and decide that life isn't too bad anymore, leave the forum, and go out to see like-minded, friendly people.
I'll admit that both are good solutions, but with the second you don't have guilt and blood on your fingers.