It's not the fact that he killed for his cause. It's the fact of who and how he killed. He promoted the idea of killing "every white person in South Africa". He killed other blacks who he felt sided with the white leaders. He killed people publically by putting a gas-filled tire around their neck and lighting it on fire.
I have no problem with someone fighting against a corrupt government. If he stuck to fighting and killing the enemy, I'd have no problem calling him a hero. However, he was a terrorist.