Sometimes being a good person is not rewarding. In fact, being a good person and doing a good deed may sometimes result in high risk to the self. In most of these cases where the good Samaritan gets hurt is when trying to stop a violent individual from attacking their victim, or trying to stop them from attacking each other.
One thing to know about bullies is that they often do not just have power because of their bigger stature or meaner attitudes, but they also have numbers backing them up. A lone bully can only get so far without the help of goons backing him up. When you interfere, you risk the wrath of the goons.
One other issue is not fully comprehending the situation and taking action against the wrong person or for unwarranted reasons.