Imagine Person A walking up to Person B's door almost knocking but hears laughing. Person A looks in Person B's window and sees Person B kisses Person C. Person A watches in shock then running away crying. 

Person B calls Person A over and Person A comes over almost crying. Person A doesn't tell what they saw but plays along with Person B like everything is normal. Person A wonders if Person B imagines Person C and not Person A.

Later that night Person B is sleeping. Person A gets up from bed and looks at Person B. Person A pulls out that gun they bought from the store and points it at Person B. 

The next day Person A running away from Person B's house with the gun still in their hand.