Writer’s Block help! What motive would you give my antagonist?

Just a little bit of info from my story: My antagonist(Charles) is a stalker who kills my main character’s parents and threatens to kill her best friend and five year old cousin. I know it doesn’t give you a ton of information, but I just cannot think of anything I really like!

Some information about my characters: My Main Character’s name is Jameson(Jamie for short), and she is a sophomore in high school. Her mom(for now; able to be changed if needed) is a rehabilitation trainer and a health expert. Her father(also can be changed) is the head sheriff of the police department.
I’ve considered revenge, and I’m thinking of sticking with it. I was thinking about my antagonist plotting revenge on the parents for something. My plan was to have the antagonist(Charles) get back at Jamie’s father for arresting Charles’ brother for drugs: drug dealing and abuse. But Charles was doing them as well; he had just left the scene for a second. Then Charles would keep doing drugs, hallucinate and think that a hallucination was telling him to kill Jamie’s parents and eventually to kill her cousin and best friend.

So should I stick with it? Is it a credible motive? What would you add to it to make it better? Just a bit of help please!!! Thanks so much!

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