A thriller, not well known, probably made in the late 80's or early/mid 90's, probably not an American film, possibly British?
A couple makes friends with an Eastern European guy who knows martial arts -- in one scene as the three are walking outside (down a street? through a park?) he kicks really high, knocking a leaf in a tree.
He starts out nice and soon turns chillingly bad after having kidnapped the couple -- it's clear they can't do anything, the boyfriend is totally out-aggressed. The plot is fairly twisty -- it's possible the girlfriend turns out to be partners with the bad guy... not sure about that though...