After Q freezes the crewman/ensign/whatever in "Encounter at Farpoint," Picard insists that he wouldn't have been harmed: the phaser was set to stun. Q then states something along the lines of, "Would you want to be taken defenseless, captain?" I forgot his exact words, but it was something like that.
In-universe, I suppose Q was playing with the crew. Q isn't always straightforward, of course. Out-of-universe, I think maybe Q's character hadn't been fully fleshed out, and this was bad writing.
I prefer in-universe explanations.