Knowledge Over Soul Essence.

That means he was attacking the soul prowler, retrieving the soul fragments, brainwashing the soul fragments, and grafting the soul fragments into himself at the same time. 

If he didn't do that then this fight would be a waste of time. The nether water will eat everything and make it so that it is not worthwhile to fight another soul prowler. 

"This spiritual dimension is too wicked." He complained as he chopped up the soul prowler trying to flee. 

"Soul prowlers can die trying to live by hunting." He said as the soul prowler shrieked and wasted more soul essence.

"This is not the survival of the fittest. This is the survival of no one."

More of the soul prowler disappeared as Nether water ate them. 

He complained again, "At least leave something behind for the victor. Don't be so greedy."