[Opinions] which is kind of a shame ...
Because:
1. Ali was named after his own father, Cassius Marcellus Clay Sr.
2. Presumably, Clay Sr. was named after a famous abolitionist and ambassador to Russia in the 1860's named Cassius Marcellus Clay.
I'm not sure why Ali thought his name was a slave name; his father was born in 1912, and presumably his parents had not been slaves themselves, and had named him after the abolitionist.
I just looked up the name on Wikipedia, I didn't know there was a famous namesake behind Ali.
"It's one thing to be open-minded and quite another to be so open-minded your brains fall out."--Dear Abby
"Let other people push you around, and you deserve whatever bad things happen after that."--Lauren Bacall