Question: Why did Argyle want to kill Michael? I can understand wanting to kill those who harmed him but Michael was an innocent boy.
Answer: While Argyle shows a strange affinity for Michael, his primary goal is still revenge on the entire town. Innocent or not, Michael is connected to those who tortured him.
How could Michael have anything to do with those men that he killed? He's just a kid.
How is he connected?
Answer: She said in the movie, "He must see himself as a child in Michael. He wants to get rid of him to erase his past."