Twisted Nerve

Twisted Nerve is a 1968 BritishAmerican psychological thriller film directed by Roy Boulting and starring Hywel Bennett, Hayley Mills, Billie Whitelaw and Frank Finlay. The film follows a disturbed young man, Martin, who pretends, under the name of Georgie, to be mentally retarded in order to be near Susana girl with whom he has become infatuatedand who kills those who get in his way.

After the title sequence, Martin is shown in a toy store, gazing at Susan, who purchases a toy. As she leaves, Martin follows after having pocketed a toy duck. Two store detectives ask them to return to the managers office. The detectives assert that Martin and Susan were working together to allow Martin to steal a toy. Susan assures them she has never met Martin. The manager asks Susan for her address and Martin appears to make a mental note when she offers it. When questioned by the manager, Martin turns soft, presents himself as mentally challenged and calls himself Georgie. Sympathetic to him, Susan pays for the toy. Certain that this was a misunderstanding, the manager lets them leave.Martin returns home and finds his parents arguing in the parlor, over his lack of interest in life. There is allusion to some perverse behaviour he has exhibited, though this is not elaborated upon. He shuts himself in his room. While secluded, Martin stares in the mirror, barechested, examining his frame. He seems disappointed in his appearance, eventually punching and cracking the mirror in frustration. The camera reveals a stack of bodybuilding magazines on his dresser. ........

Source: Wikipedia