Yeah, I was wondering the same thing. I guess Law has some liking for the Straw Hats, seeing as he went out of his way to save Luffy when he didn't have to. At the same time, he also said, "I can't let them get away either." Obviously he wants to confront them again, but for what reason? If the Straw Hats see whatever secrets are on this island, they would obviously be a threat to him and become a liability to his plans.
There's also the obvious connection Oda set up with Vegapunk going on, Franky's knowledge of Vegapunk from his winter island, and some other things Oda seems to be lining up for reveal and/or hint in this arc. Good stuff. I already can't wait to see these chapters animated, but that's a ways off.
He seems to be about as hostile to them as he is to marines, and different attack could be just for the sake of variety since none of his "attacks" are exactly lethal anyway.
I imagine switching people to such drastically different bodies would have about the same effect as slicing - they are non-combatants either way.