I think there is a small (but not that small!) part of society whose experience of the people is entirely negative,who have seen the police abuse, threaten and manipulate evidence against their friends and family.
I'd like to say it's not all police and it's certainly not in all cases, but if you are poor and struggling in an area of high deprivation and social issues, the police are not your friends.
Obviously no one can condone violence against them, but I fully understand the view of some of the people I support in my work who "hate" the police and see them as the enemy.
My experience of dealing with them (very rarely) in my own life is very different to what these very vulnerable young people experience.