Robin Williams did more in his lifetime than probably 4 average people. Think of all the places he has seen, the parties he has been to, the amount of money, the amount of people telling him that he has made their life better or whatever.
Dude seemed like he was ready and determined to die. If someone is ready for death, I don't generally feel sad for their passing. Robin Williams will never hurt again. He will never have pain. He will never feel depressed.