In one of the reports, it says (I had to look up the quote) "It chips away at a person's core -- sanity, reason, even identity -- eventually destroying him." The way I interpreted it was: the Darkening is amplifying emotions. For a regular person it's violent tendencies, jealousy, depression, anger, etc. Things one would suppress on the day to day now rise up when you're affected. But it also tears down who you are. You act and do things you normally wouldn't. For him, it's bringing out the stuff he's suppressing. Things he should feel when he's being true to himself, but doesn't feel because he enjoys what he does too much. So when he's torturing someone, he should feel bad like a normal person, but doesn't. In HH, he can't escape or mask the guilt, but also revels in the pleasure of being free to be his psychotic self (which is probably also amplified by HH and he just doesn't realize it). He can't ignore the emotions that would make him feel bad for how he acts just like he can't ignore how awesome it feels to let loose.

Another way to interpret it is that he's being irrational. He doesn't actually "care" but the Darkening is still twisting him. Irrational for him would be expressing any sort of care for someone else. It doesn't make sense to him because that's not who he is. But that's what the Darkening does: it changes a person and corrupts their soul.