There are five different possible answers to this.
A) He is delusional
B) Glaciate works differently on Humans then it does on Pokemon
C) Glaciate kills people slowly, so we would be left alive to watch him rule before finally dying D:
D) He is now so deranged that he no longer retains his senses, so he figures he would freeze you, which would actually kill you
E) He does not actually know how powerful a move Glaciate is
Although all of these are myths, there is one fact that Ghetsis is mental in BW2, like literally. This may be the reason, and is what I find most truthful, although you never know :3