No no, that's the reason why he lost his familiar powers in the first place. Because he technically died during the 1 vs 70,000 person war.
He didn't kill one person but knocked unconsciousness a large portion of the army, mainly going against commanders. He was striken down eventually, and due to the army considering him very honourable etc they gave him a heroic killing where the entire army were to fire at him at the same time.
In the story, at this point Saito was already dead, but then Derflinger took control of Saitos body and high-tailed it, ended up in a forest, Elf saved him, dying cost him his familiar powers.
As I stated, Louise simply just attempts to resummon a familiar and Saito gets his powers back.