Has anyone ever been killed on Mythbusters?

Answer 1

Yes. The Mythbusters themselves have answered this question on their website. Here are Adam and Jamie's answers.

Adam- "Besides a couple of stitch-worthy cuts that I've sustained, I'd say the greatest injury has been to my dignity when receiving a rectal thermometer during the "Goldfinger Revisit" myth."

Jamie- "So far we have only had minor cuts and scrapes. The worst of these was a broken finger sustained, ironically, by one of the crew when handling safety equipment; specifically the bullet resistant Lexguard panels we use which are quite heavy. We are becoming increasingly aggressive about maintaining safety on the show as over time -- as we are often replicating circumstances in which someone got hurt or killed, let's just say we have reason to be cautious."

Answer 2

No. There has never been a death on the set of Mythbusters. Many security precautions are taken to ensure the safety of everyone involved in the tests.