This is not exactly an AI like we are seen right now, but there has been at least one instance where a robot killed a human in 2015.
https://www.theguardian.com/world/2015/jul/02/robot-kills-wo...
https://www.theguardian.com/world/2015/jul/02/robot-kills-wo...
Factory robots like this have no decision making ability, so it’s unrelated to potential AI killings.