Cannes: A tough and unflinching social drama about the predators among us — and how difficult it can be to see them for what they are. Marie Kreutzer ’s “ Gentle Monster ” opens with virtuoso musician Lucy Weiss ( Léa Seydoux ) sitting at the piano in her Munich apartment and plunking out a stilted but affecting cover of Charles & Eddie’s neo-soul classic “Would I Lie to You?” This is before her Austrian husband Philip (Laurence Rupp) — a documentary TV producer of marginal success — comes home