I’m trying to find a film from the 70s, british, I think. A blonde child gets adopted and one by one starts killing each sibling mostly while on a family picnic. The child eventually kills the father too. All to have his mother all to himself. At the end he end up with just his mother.
The Godsend
https://www.imdb.com/title/tt0080802/?ref_=fn_tt_tt_9