Frank Mir claims he wants to kill Brock Lesnar in the Octagon in an interview with Mark Madden

Not really, as Zach Arnold points out it's just a misjudged pro wrestling style promo to hype up a potential rubber match with Lesnar.  Of course, a trained professional wrestler would know never to say such a thing, because you never tease something you can't deliver on.  In MMA it's even dumber, given that the sport is not yet legal in some major North American markets (such as New York and Toronto) and opponents of the sport could capitalize on such comments to keep it banned in those states.  And Jonathan Snowden was mocked for having the gall to suggest that such promos could be bad for the image of the sport after UFC 100!  Fortunately no-one was paying attention this time, so hopefully Dana White will give Mir a good talking to and it won't happen again.