I read this book two years ago during my freshman year so I may have forgotten a few details. Hope this helps though.
Answer:
Question 1:
I think the citizens of Macomb saw boo as someone from another planet. He was thought of as a creepy outsider, even thought of as a monster to some of the kids. His backstory seems scary and suspicious which gives even more reason for the towns people to have a negative opinion of him.
Question 2:
When reading chapter one, we find out that as a boy, Boo got in trouble with the law and his father imprisoned him in the house as a punishment. He was kept in that house for fifteen years until one day when he stabbed his father with a pair of scissors. It was not his choice to be locked inside his house.
Question 3:
I think that Boo has been unfairly judged. Since the whole town is afraid of him, I think later on he will try to be a normal citizen but people will reject him and misjudge his actions before they know what he is doing.