Eh.... I don't know if the US has been around long enough to really grab that title. It kinda depends on who you are laying blame to for the vast majority of the genocide against native Americans. Most of the deaths during the great dying in the Americas happened before the concept of America as a country even began.

If you're looking at the amount of people killed and adjusted for the comparative populations, it's probably the Spanish empire. If you are looking at the number of different cultures affected it's probably the British. I would say America wins the most evil modern empire, but history is pretty brutal and America kinda missed the majority of the age of exploration and the colonial era.