Depression is a mind-altering disease, so no, they don't have complete control over their actions or feelings. Mental illness poisons everything you do and prevents you from living your life to same extent that mentally healthy people can. They have a voice in their head constantly telling them that they're useless, worthless, pathetic, will never amount to anything, should just die. They do not simply go, "Drat, I got a bad grade. I'm gonna kill myself." Depression is a serious and complex illness, and as I said in my original comment, you should educate yourself more about it before saying such hurtful things.