Because that's such a crappy end for Mirror Master :(
Motto. Particularly because, as Morrison wrote him, McCullough (after his first appearance, anyway) was much more of an anti-villain than the usual completely sociopathic villain type. The man had moral standards. Although he had no problem beating up Buddy as a warning (and rather enjoyed it, too), he balked at killing his wife and children, and when the evil business cabal hired someone else to do so, he tried to warn Buddy, and when he couldn't, he helped him get revenge (and was even shocked at just how far Buddy took it.