“Or, as likely, boy dies because man pushed him down stairs. Girlfriend knows. Man kills girlfriend to silence her, but tries to make it look like a suicide.”
Or, boy falls down stairs, girl feels responsible, hangs self. Suicide is the most sincere form of self-criticism.
Or the place is haunted. I could go either way.
Or, girlfriend pushes boy downstairs. Man finds out she did it, freaks out and kills her.