I've never had anyone very close to me attempt or commit suicide, so I don't know how it feels or what the signs leading up to it might be, but unless someone just wakes up one morning and decides that day to off themselves, I would hope I could pick up on their despair and divert them from this saddening, unnecessary act.
I heard that Mr Williams had been depressed for some time. To me that doesn't explain it. That just tells me that there were definitely signs and things those close to him could have done. Of course, I never knew the man personally, I'm just a fan of his work, to put it mildly. So maybe he was just doomed to this end, being depressed all the time. He wasn't as fortunate as us to have his own Robin Williams to make us laugh and cheer us up. Rest in Peace, friend. For your sake, I hope it was just an accidental overdose.