Board Thread:General Discussion/@comment-29546017-20160809203518/@comment-26887879-20160810161208

You can kill a ghost only when they are "fused" with body they posessed. Mad Dummy is also a ghost, but you also can't kill them because they aren't fully "fused" with dummy they posessed. My guess is Mettaton gets fully fused with his robotic body once he goes EX or NEO, so that's why you can kill him.