Tim Roth wanders through “Sundown” like a man whose mission is avoidance: of responsibility, of expectations, of life. He plays Neil, a man on vacation in sunny Acapulco with his sister Alice (Charlotte Gainsbourg) and her two teenage children (Samuel Bottomley and Albertine Kotting McMillan). They’re living the good life at a posh resort when Alice suddenly gets a phone call that their mother has died. They rush to the airport to return back to London but Neil doesn’t board the plane. He forgot his passport back at the hotel, and he’ll grab the next flight out. Except he didn’t, and he doesn’…