The cop should not make arresting someone his first priority in an accident scene. He should first determine whether the accident victim needs medical attention and if so work on that problem first.
Whether an arrest requires force is another topic all together.
In other words, in this particular case if he had fulfilled his first duty violence would never occurred.
Sadly, police departments are rewarded with money for every arrest they make.
If you monetarily reward for arrests made, they become biased to look for reasons to arrest people for money.