It pretty much is that simple. Two cops shot a 15 y/o kid. These people rioted against the WHOLE government because of it. Even though both of those cops have been arrested. They even rioted at the kids funeral!?
Throughout the last 3-4 decades, Greece has witnessed clashes with civlilians and law enforcement. There have been a number of cases in recent history of law enforcement brutality. One in particular was a couple months ago when a group of cops searched a suspect and made him undress almost completely in public. To sum it up, cops and civilians haven't been on good terms for decades. There have been cases of gang fights around cities while cops are dining out and not responding to calls. Things like that.
Enter recent increased job unemployment among youths amidst the financial crisis, very unpopular education and university reform, and financial bailouts for banks that people think banks don't deserve.
All of that culminates in these riots. The shooting of the teenager just made the riots worse.
Also, none of these riots started as riots. They were peaceful student demonstrations that escalated into riots. Like most demonstrations do because of the way the police respond to these demonstrations.
See now why it's not that simple?