It's still not clear why they would enter a BK (full of tear gas, according to the journalist) while at least some of the surrounding roads were free.
If they really were beating innocent bystanders then they should be punished to the full extent of the law, while at the same time I understand the violent reaction of the police after having been pelted with rocks for hours just for wearing a uniform.
I've been in these kind of protests before, you can tell well in advance that things are going to get ugly. When you have police being attacked from all sides and you intend no harm, you know it's time to gtfo because you will get caught up in the violence.