"He was a lot of things to me" "The only conclusion was love"
He may even be a bit scared of her himself, and feeling like the Charmings need to step up their game for his sake as much as for their own. In which case, being a prick in that scene is actually his attempt to help them all.
I think Rumple is VERY scared right now. He almost just died and at the hands of Regina’s mother no less. And when Rumple gets scared he goes Dark Side (threatening to kill the Charmings for instance after losing Belle). It’s the only way he knows how to cope with fear because he truly believes that fear and cowardice are a weakness.