In any group of people, you will find a bell curve distribution. There will always be a few under the left end of the curve who pee in the pool and ruin it for others.
Let’s turn it around. You listed a handful of incidents and used them to declare all police as being untrustworthy. There are nearly 63 million contacts between citizens and police a year (original figure from 2011, revised 2016. PDF here.). If we take out your contacts, that leaves us with 62.8+ million contacts where the police did not run amok and slaughter people indiscriminately as you would have us believe they do. You stand a better chance of being hit by lightning (1 in 700,000) than you do of being shot by a cop.
Yes - we should trust them. Deal with it.