What's holding them back from madness while using the power of it? Kid and Black Star seemed to be in fine control of themselves and simply wanted Maka to give them a power boost with Chain Resonance. It doesn't quite make sense for this infective Madness that they've been trying to eliminate for so long to be an actually harnessed power.

It'd make more sense if they were constantly being held back from madness takeover by having Maka and Soul keep it on a leash via Chain Resonance instead - Claymore style.