Alex Johnston on 4. Dec. 2017.
The link below explains everything. Sargeras originally protected the universe from demons and void energies until he realized that the void lords are actively trying to create a titan that has been enfused with their magic by using old gods to corrupt slumbering world souls. This is why you see him stab Azeroth, as he believes it is corrupted and an evil titan will emerge and destroy all of creation. He may appear to be the villain on the surface, but the motive for his actions is noble.