[PLEASE Help me find the name of this movie!!!!!]

I cannot thank you enough!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! I have been searching for this movie for months!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! You are WONDERFUL!!!!!!!!!!!!!!!!!

[PLEASE Help me find the name of this movie!!!!!]

I am not positive if I have all of the plot right because I didn't get to see the whole movie. Man A worked in a drug store or general store. He was a little quiet and nerdy. His wife, or girlfriend, began seeing Man B. Man A changed his identity, got a new apartment and a new girlfriend. Man A was planning to kill Man B, but somehow Man B ended up dead anyway. The police began to suspect the man that Man A changed his identity to. I didn't see the end. PLEASE help!! I want to see it again!! It is an old movie. Maybe 50's or 60's.