Lets look at the United States' rap sheet.
2. War crimes (see Iraq). Remember the photographs of Iraqi prisoners of war being forced to crawl down the hall naked as dogs nipped at them? Remember the pile of naked prisoners? Remember the hooded prisoner with electrodes clipped to his extremities? By any rational standard these were war crimes; clear violations of the Geneva Convention's rules regarding the humane treatment of prisoners of war.
3. Guantanamo Bay. Speaking of the Geneva Convention, holding prisoners of war indefinitely after the end of said war (we are leaving Afghanistan are we not?) is illegal. Simply declaring that the prisoners are not prisoners of war, but not criminals either, and using that as an excuse to violate every standard of humane treatment, is not permitted under international law.
4. Torture (see Guantanamo Bay). Although water boarding is the most well known form of torture practiced by the United States in Guantanamo and elsewhere, it is not the only one. Multiple "harsh interrogation" methods used at Guantanamo were publicly considered to be torture by United States. That is, until the news came out that the U.S. was using them. For the record, torture is a violation of international law!
5. "Rendition" (see torture). Rendition is a extralegal process of kidnapping someone, often in a foreign nation, and holding them in a secret prison for interrogation. Often this interrogation involves torture, sometimes in a third party nation with lower standards than even the United States. The most famous case of this involved a Canadian citizen detained in New York and sent to a torture chamber in Syria. Yes, Syria.
6. The drone war. As a general rule, it is against "international norms" to embark on a bombing campaign in half a dozen or more countries with which you are not at war. Call it "targeted killings" if you want, but when a unmanned drone drops a bomb on a residential neighborhood, that qualifies as an act of war by international standards. Having a drone drop a second bomb minutes later to kill "first responders" is also a violation of international "norms".
I could go on, but I think you get the point. We in the U.S. like to think of our country as a model citizen of the world community, but much of the world views us as a rich, arrogant bully; a mafia don who is despised and feared, not respected. If there are "good cops" and "bad cops", the U.S. is that cop on youtube beating the crap out of some defenseless suspect that is already in cuffs; the Texas patrol officer feeling up a female driver on the side of the highway; the cop slamming the 83 year old lady onto the pavement in a parking lot because she had a butter knife in her hands.
Policeman of the world? We are an outlaw nation that has no business writing parking tickets, much less enforcing international laws which we have shown total disregard for.