Jealousy
There are a lot of emotions that we feel, some are good and some are bad, but that is just what makes us human. We should listen to our feelings because they are trying to tell us something, but we should process them in a mature and healthy way, not like this guy.
He clearly has some issues and doesn’t know how to manage his emotions, so he took up violence and murdered his ex-girlfriend’s boyfriend. Sounds like an awful situation to be in and now he has a kid that doesn’t understand why any of this happened. What a shame for everyone involved.