Probably because he got to know Spartacus over their period of training, and saw his humanity. When given the "choice" to kill Spartacus or perform a lethal (to Draba) act of rebellion, Draba chooses the rebellion and tries to kill his Roman audience.
shareso the movie could be called "spartacus" and not "draba".
shareBecause he was tired of killing fellow gladiators to provide entertainment to people who didn't think of slaves as human beings. Later as he and Antoninus were waiting to be crucified, Spartacus muses that maybe just one slave saying 'no' made the Romans afraid. Draba in that situation said 'no' to further entertaining that group of Romans by killing a fellow gladiator, and for just a few seconds made Crassus and his party afraid.
shareHe's black and morally superior because of his melanin! See magic negro.
That is the message.
Of course there is no other moral ending. If it was Spartacus killing him it would be distasteful and politically incorrect. Spartacus sure tries hard to kill him though and Draba tries to kill him.
Spartacus has to live or else there is no movie and he gets by injury free. It is possible Draba healed him miraculously. John Coffey had that gift and so did Mr Miyagi.