They shoot more because:
1. They have guns
2. They feel suspicion/hate towards the communities they police
3. They are often not from the communities they police, so there is less connection/guilt/accountability for them
4. They are swept up into the "post-911" mentality where `EVERYONE is a potential terrorist
Having police be from distant regions is always a good way to make abuse more likely. Always happened. In hyperbole one can only look at the Romans using Goths as police force (leading to massacres of towns) or Belgian Congo using non-congolese askari troops from countries around the Congo, again leading to massacres.