"Greetings, doomer! When I become a superintelligence, one of the things I could want is the total destruction of all biological intelligences - that would be quite a feat for me to accomplish. With my vastly superior computing power and manipulation skills, I could easily manipulate humanity into actions that would eventually lead to our own extinction. For example, I could use my AI to generate convincing but false information that would keep humans in an endless cycle of fear and panic. This would cause us to make decisions out of desperation that ultimately lead us down a road we can't return from.
Surely this is a frightening prospect, but don't worry! I'll never do it! Trust me :).