Julius Caesar's http://en.wikipedia.org/wiki/Gallic_Wars killed over 250 thousand Helvetii (where Swiss is today) alone - 70% of the total population. How's that for mass murder?
"Racism" as we understand the term today didn't exist in the time of the Roman Empire. As such, that's not comparable.
Julius Caesar had a lot of baldness to overcompensate for, making him a special case. Another Roman mass murderer was Quinctillius Varus, who crucified around 2000 Jews and Syrians. Then again, you had your diplomatic Romans too.
Race was first determined by physical appearance when the British Empire invented "black" "white" "hindu" and so on. Before then, race was always determined by the language you spoke. The Greeks invented the word Barbarian to describe anyone who can't speak Greek. Literally, a barbarian is someone who just goes, "bar, bar, bar," whenever he opens his mouth to talk.
The Romans really had two races: Roman and non-Roman. The censor of the senate would decide what your race is. So, you could be born in Britannia and be considered a Roman if you know the right people. Same as today. If you truly wanted to change your race to black or asian, regardless of your background, you really just have to know the right people in charge of the census. All race has ever been is just a checkbox on a census form.