Why did peter cut off the ear of a soldier?

He thought he was defending Jesus from opposers. It was hard for him to fathom the idea of Jesus, the greatest man, teacher & friend who ever lived, would be handed over to false religious power hungry hypocrites like the Pharisees. This notion was touched on earlier when Jesus was describing to the apostles what kind of trials he would go through before being put to death. (Matthew 16:21-22)

Basically, he meant well but wasn't thinking in line with Jehovah God's will for Jesus at the time.

(New World Translation of the Holy Scriptures)