I can’t find the title for this movie!

When I was younger I saw a movie about a guy knocking on a woman’s door and pretends to be her abandoned son, while her real son is in jail. The woman lives with her husband and teenage daughter.

Later on they invite him to live with them because I thought he was pretending to be a student, but he kills the husband (I believe it was with a syringe) let his blond crazy girlfriend keep the daughter hostage with a gun in the basement while he forces the mother to take a shit load of money from the bank at the end.

I have been searching for months but I can’t for the life of me remember what movie this is. I was young so I couldn’t remember the actors.

Please help me.