Part of it is training but part of it is also what police officers face. In my country the odds a police officer will pull over someone for speeding who has a gun in the car is 1:10000. In the US in some parts I wouldn't be surprised if it's more than 1:10.
If you then look at inequality, mental health issues, crime rates etc... which are largely a function of socioeconomics, man I wouldn't want to be a police officer in the US, either.
It's not necessarily the case that I love the police officers in my country (north-west europe), either. They lie, they don't enjoy it when you verbally challenge their power with an argument, they're fallible.
For example at one point I saw an officer harassing some homeless guy, asking for ID etc. I know they're not looking to help them get shelter, but just looking to get them out of the park because people complain. So what they do is they ask for ID, they obviously don't have one, so they put 'em in a police van, hold them at the police office overnight and then release them outside the city centre the next morning. If they do it often enough, they stay out of the nice city centre where the rich folks live. So I talked to the officer about it, but you immediately get told to stay out unless I wanted a problem: i.e. arresting for obstruction of justice, held and later released. There's very little you can do in this situation, either you shut up or you have to call work and say you've been arrested and can't come in etc. It's not the end of the world but it's an effective deterrent in many cases for having small injustices go unchecked.
Same with racial profiling, happens all the time as well. And every single time I've reported a theft, they did absolutely nothing about it. Even if I gave evidence of the crime and the person's identity etc. Lack of resources... It's understandable to some extent, but frustrating too. It almost invites you to get your own revenge if I didn't know better.
So I don't love the police, either. Police officers are not all that different from most people in their human flaws, except that they're typically relatively lowly-educated (and in the US, lowly trained), combined with far reaching powers and an institution that backs them, it can be problematic in any country.
But police violence is very rare, police drawing guns is rare, let alone shooting them. People in all countries have issues with their police (like they have issues with bad teachers, bad doctors, bad judges, bad politicians, bad hairdressers and bad chefs!), but within OECD countries it's basically only the US that has this level of (lethal) violence. Guns play a large role, as do socioeconomics. But there also has to be strong institutional oversight, it seems the police is institutionally far more inoculated from the consequences of their actions in the US than elsewhere.
Where I'm from, every damaging use of a gun by a police officer will get an automatic investigation, not internally but by an independent national research team that falls under the national prosecutor.