Mid 70s Horror Movie

[hotspur]

I was too young to see it, but I remember a movie being advertised in the mid 70s that was a horror movie.  I think the idea was that one person (likely a child) was evil and killing off his relatives.  In the trailer they showed one person being sealed in a room, with voice over saying he (the villain) gave them (the victim) a room of his own and another scene where 2 people are crushed together in a shower.  Any ideas? 

[zombie no.one]

nothing springing to mind... I guess PSYCHIC KILLER ticks a few of those boxes. not child though.

[Trevor]

I was wondering if this was ARNOLD from 1973?

https://www.youtube.com/watch?v=I49SGOurZTw

The "room of his own" and "the crushed in a shower" scenes are in this trailer.