Both of them are true. Being policed will cause resentment on its own, even in most clear cases of the police doing a good job. A son is being charged for a crime that they say they didn't commit? Even if there's evidence to show that the son committed the crime, in many cases the family will start resenting the police for that. At the very least subconsciously, but you also end up with enough cases where people simply want to believe that "Our Little Jimmy is not a criminal!" and go into denial, even in the face of clear evidence that he is.
Now, such cases are local, and normally, the overall impact of that would be a positive one. The family may start resenting the police more, but everybody who looks at the case reasonably will think they did a good job. However, add a high crime rate where many people have such cases in their families, where most people hear anecdotes of police arresting people who "were innocent", and your overall resentment of the police will go up significantly, because you get to a point where these stories are so common-place that people start doubting the story put out by the police. Add a racial element, and the "us vs. them"-mentality gets to a boiling point where even a police force that makes no mistakes, and does their best to keep people save, may be seen as enemies by parts of the population living in those areas.
The fact that there are officers who are badly trained, racially motivated, and/or very confrontational magnifies that problem significantly (and gives validity to the resentment of the people), but it would still exist in areas with high crime rates, even if police didn't make mistakes..