My Picks:
I'd argue the two biggest crimes in American history are the genocide of the Native Americans and slavery. ("Honorable" mention: the Japanese-American internment camps, but "at least" those were comparably short, and not as many people died...which is more about how awful the former two are than anything else.) However, I'd argue that you can't pin the genocide on any one figure, as it took place over hundreds of years, across more liberal and more conservative Presidencies and Congresses...it was just a society-wide disgrace.
On that note, I'd pick the leadership of the CSA as the Most Evil Americans. The Confederacy, defending slavery long after it should've been abolished, breaking up the Union, and spawning later and persistent evils (see: the KKK.)
Most Evil Brit...
Not my country or history, so I'm obviously not as qualified to make a pick, so please correct my choices if you think them unfair, British WebDippers...
Oliver Cromwell seems a nasty figure, almost a proto-Robespierre, ie, setting up a revolution for more democratic rights, only to become a tyrant, and his actions vs. the Irish are hotly debated, but could be held against him as well...
Then again, he also allowed Jews to return to England, and Edward I, for this expulsion of the Jews, would be another partisan choice of mine.
There are probably better choices, and nasty figures from when the West was, you know, carving up the world...again, I'll let someone more qualified than I get this one going properly. :)