The Trainer 2013 on netflix

Annie, 28 and recently divorced, is doing everything she can to get her life back on track. When she decides to join the New Life Fitness gym she’s inspired and excited about her new trainer Alex, 20-something, who quickly becomes a friend. Alex is young successful and full of life. But that success comes at a price. Sadly Alex is psychotic and has been murdering her trainees for years. Turns out working out really can be murder.