Captain Preston was guilty because they fired upon disorganized and unarmed people so I believe they are in the wrong. Captain Preston says that the soldiers were confused but if he was right behind them then they could clearly hear him say "fire". So, Captain Preston was in charge of his men the whole time and that means that he was responsible for the incident.
It is because the Americans started to see all the other countries become free and they wanted to become free to. They also started to not like the way the British treated them.
