Actually in Overlord there is a undead that can passively devour the souls of those close to it and yes Ains resist it.
Also Ains could not do the same to Shalltear because she already was an undead.
So depending of how we consider the 6 paths Ains will have 2 options:
1.- We consider them as undead and Ains know that from the beginning via his passive and don't waste time trying Instant death attacks
2.- We consider them as corpses and If Ains were to discover that he ends the fight transforming them into Death Knights
Also Ains could not do the same to Shalltear because she already was an undead.
So depending of how we consider the 6 paths Ains will have 2 options:
1.- We consider them as undead and Ains know that from the beginning via his passive and don't waste time trying Instant death attacks
2.- We consider them as corpses and If Ains were to discover that he ends the fight transforming them into Death Knights