But this is not the most interesting question to ask. In the US, a disarmed police force would probably just carry privately, while on the job.
A much more interesting question is why so many armed police forces in the world get by with a level of violence not so much different from their non-carrying peers. The U.S. traditionally condemns many nonviolent police activities (like searching, or requesting ID, which theoretically does not even exist there) as oppressive, anti-freedom, unamerican. Other countries do not have this cultural expectation, that the police should never interfere with the life of non-criminals. Thise countries therefore have a wide, nuanced range of police/citizen interaction that only involves guns as the very last level of escalation. If culture (and/or law, but I think that culture is much more important here) only acknowledges police authority stemming from a pointed gun, officers will find excuses to point guns.
But the biggest reason for US police gun violence must be the high general level of gun violence. It seems very natural that when criminals are trigger-happy, the police force adapts by lowering their thresholds accordingly. Inevitably, this will also fatally affect law abiding citizens. I haven't checked the numbers, but maybe, in relation to total gun deaths (instead of in relation to total population), American police are not even as terrible as they seem?