because he had the chance to kill Tsunade and the other kages but he didn't do this
Do you wonder how this is a good reason?
1.Killing Tsunade would have opened the door to the hokage seat and that's his current goal.
2.Madara left them all half-dead, sasuke could have ordered oro to finish them off and none could know that sasuke is the one to kill them because Madara already told Naruto that they are "not okay" so it was announced that they are dying.
3.he knows the kages hate him and would refuse him being hokage but still didn't want them dead.(if he was evil he could have let them die)
_Don't try to discuss "he wants to kill naruto" topic because it's the only plot reason for them to have a final fight.
_Finally, sasuke may have bad traits but being EVIL is not one of them.