The blood-bender is Katara.

Sweet, innocent, old-ass Katara.

Could it be that Katara wants to present Korra with a worthy adversary? Could it be that Katara believes that normal people have become second class citizens and the benders need to be culled? Could she just be doing it because she's old and depressed that everyone she's ever known is dead and buried? Who knows...

Whatever her motivation could have been, the idea that one of the heroes of the previous series which showed her blood-bending twice and also showed that she was tempted by the power, could fall prey to it would have been chilling and, honestly, no one would have seen that shit coming.