Execution of the thief?

Yes, it's something normal and happens very often down here in Rio. Private security guards and police officers do it all the time, especially in the pooriest areas of the city. It's obviously illegal. The kid was supposed to be taken under custody, but some cops just won't do that if they have the chance. They'd rather to observe the "due process", if you know what I mean: extorsion or summary execution. Or maybe both.

However, I can tell you that that particular scene is pretty unrealistic. First, if a thief was stupid enough to steal a ambulant salesman at Rio de Janeiro Central Train Station in rush hour, the last place he would run to is the railroads. Besides, the guards would never kill him in a crowded place like that, with lots of potential witnesses around.

Swift justice as Judge Dredd would say.

It's that man again!!

I guess it's not that uncommon in USA as well, with all the police brutality observed especially in racial profiling.