so, the repeated shots at close range are clearly unjustified. it's excessive force, and the officer should be charged with and tried for murder and go to jail for the rest of his life.
however, given that the guy got into a fight with a handful of cops (he seems to have been trying to prevent somebody from getting arrested, which he shouldn't have done. he should have kept filming and offered it as evidence.) and had a gun on him, the officers had grounds to use some amount of force, such as shooting him in the leg, in order to stop him from resisting. the officers knew he had a gun. he brought a gun to a protest. why? did he have explosives? other weapons?
there is no justification to shoot him repeatedly in the head, but there is justification to use force, and he should have been arrested and charged. he cannot help his friend by getting into a physical altercation with police. that won't prevent his friend from getting arrested. he can only put himself in danger and can only irrationally break the law. if he thinks his friend is being arrested for civil disobedience, and thinks it is illegal, the best way he can help is to document it for his friend's defence.
being a cop is hard. that's not an excuse, and this cop needs to go to jail. but this is not the same thing as what is happening in iran, at least not yet.