Feel remorse for his murders? ....They are trained not to feel emotions when killing.. they are ninjas , hired assassins.. this is why he mocked sakura for stating something as silly as that..The ninja world isn't filled with mary sue characters..... On the topic of evil, evil is a matter of perception, based on different people they will have differen't outlooks on the word....The thousands he slaughtered on the battlefield and for akatsuki were justified acts of war.. the hundreds he had under mind control as spies, would be another justified act for espionage purposes.. something ninjas do ..... It seems the key thing here is whether his human puppet ability would be acceptable even in Naruto terms...... The answer is simple.. What is evil? the simple definition offered by dictionaries lead to the belief that it's changes based on the person having the thought ..... Would akatsuki members, eg deidara, who are fighting for their cause, as Konoha fights for their own.. see Sasori's actions as "Evil" ? They wouldn't... Did the villagers of konoha see Tobirama as evil for creating Edo tensei? Most likely not... Did kabuto see Orochimaru as evil for testing on thousands of humans and babies? no...
From our human perspective , they all are evil... evil is the state of profound immorality , and ninjas are the epitome of immoral.. they lie, cheat, steal, kill at order and more....You can't use our perspective when looking at them, it doesn't work