ISIS is a bad group. They are expected to be bad and wild by everybody. Do they ever do any good? Even members of their own group expect that they may be unexpectedly "sacrificed" to some sudden violent project that the group decides on. If ISIS kills people, it's expected. If ISIS beheads people, it is expected. If ISIS uses suicide bombers, it is expected. They do these kinds of things all the time. Everybody expects them to do these things.
Cops are good people. They are expected to be good and self-controlled by everybody. So, when you see this group of people in blue doing things that ISIS is expected to do, all the while that cops are EXPECTED to be good, that makes them way worse than ISIS.
Cops may not be worse than ISIS in the sense that they are literally out there randomly killing and maiming and destroying. But then, they are expected to be doing the complete and exact opposite.
ISIS is like a wild bulldog, that tears anything and anybody up if it can. Its owner keeps it chained and penned up if he doesn't put it out of everybody's misery.
Cops are like the sweet, well-trained house dog who has been bringing the newspaper to his master for years, helping the family, and doing all kinds of good things. Cops are like this dog suddenly gone rabid when you least expect it.
You know you can't trust ISIS. But don't trust a cop, because he just might turn on you when you least expect it.
You and your best friend in a street gang meet a couple of guys in a rival street gang. You expect that members of the rival gang are going to fight you... could fight with you at any moment. But when your good buddy, one of your own gang members, turns on you suddenly and unexpectedly, who is worse? Members of the other gang who you expected to be against you? Or your own good buddy who you have trusted with your life at times?
Cops are worse than ISIS.