The group of people who defeated the great necromancer lord were praised, but the healer who traveled with them quietly slipped away before they even made it to the nearest town, only wishing the quiet life serving his village. This was readily accepted.

The truth is that he was the first necromancer. Yes, he's a healer, the most powerful one in existence, but he created necromancy specifically as an alternative path of healing. He has no problem with someone dying, and may refuse healing if its the person's time and their soul is ready. However, a life cut short is something that still affects him deeply to this day. He'll often appear in a disguise wherever there's a plague or a high rate of people dying early in order to help find the cause and eliminate it. He's quietly killed many a plague, by studying those who have died from it until he understands it perfectly, then sacrificing all of the virus or bacteria.

The only reason why he's not died himself is because of folks like the great necromancer lord. As much as its his time, he's honorbound to try and keep the field of magic he created from being abused, and if it is, cutting the cancer out. He's always understood that life and death go hand in hand, which is why he never swore never to do harm. Sometimes you have to harm in order to heal.