Why Did Vilgefortz Kill His Comrade

Why did Vilgefortz kill soldier?

Vilgefortz killed him not because he witnessed the latter’s humiliation, the mage was too occupied fighting the Nilfgaardian soldiers, was wounded and asked Vilgefortz for help. Vilgefortz killed him because he is a traitor and has always been an ally of Emyhr.

Is Vilgefortz a traitor?

Vilgefortz retreated, not only hunted by the Northern Kingdoms and the newly founded Lodge of Sorceresses, but also by Nilfgaard because he betrayed Emhyr by delivering a false Ciri (needing the true one for himself).