Carl's excuse for keeping the ring — that he simply will not use the skill on it.
When Mordecai demands the ring come off, Carl's first argument is that he is not going to use Marked for Death at all and is only wearing the thing for the five percent to his stats. Mordecai's answer is that possession is the problem, not use: the skill is what the ring is known for, and the crawler-hunters of the sixth floor track ring-bearers precisely because a charged Divine Suffering artifact turns every kill into permanent power for whoever holds it. The skill goes back into inventory with the Night Wyrm's Ring, unused.
The player-killer skill attached to the Night Wyrm's Ring.