Old movies related to one understandable killer or some killers? ?

Obession with Cliff Robertson, is a great mystery........ as it's the story of a simple scam that goes wrong.......resulting in death. 

The intent wasn't to kill them......but they ended up dead anyway . 

Orrrrrrrr,  did they ?    Great movie. 

another good movie about how things go too far and result in a death, is  Alpha Dogs,  with Bruce Willis. 

(but it's one of those  "Bruce Willis" movies where he barely makes little more than a cameo---he does LOTS of those) 

It actually starred Justin Timberlake.