While his Corruption heal can be used in that way, it's actual power is to transform the body from a functioning one into a non-functioning one, and setting that as a default, meaning that it can't be healed. For example, at one point he used this to block the heart's arteries of an enemy, and since this form is set as default, no amount of normal healing magic can cure it because the body itself recognizes it as "correct." At another point he fought an enemy with high-speed self-regeneration, so he used this to take out his limbs, and as the body itself recognized the lack of limbs as normal, they couldn't regrow. Also, since "healing" magic is recognized as buff of sorts, magic resistance can't block it. That's what's so scary about it.
I'd say his powers are better explained as biological manipulation rather than healing.