Spiked drink drown in a bathtub

A perfectly happy couple get new neighbours the floor below and the new people seem suspiciously creepy. It's kind of like Roman Polanski's Rosemary's Baby (1968) or The Tenant (1976) all over again, copycatted a few thousand times before and it's always a bit exciting and terrifying. This one took twists and turns that are particularly vile and evil.