Sounds like the police acted appropriately in this case, sure doesn't sound like a pleasant incident, but sounds like they were just trying to ensure he didn't harm anyone else or himself till he could be taken somewhere for an evaluation. I do however feel a bit sorry for the kid if he did act out like this due to suffering bullying he probably needs some help and support with that. Does not justify it, even at 7 most people know its wrong to attack someone because they upset/angered you...but yes having suffered much of that growing up myself it can be hard to keep your cool all the time when you don't really have any support with it.
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Metal never dies. \m/