i guess healing magic can be used for destruction as well, if you want to remove tumors i think you need to destroy them so it should have destructiopn features
here's some of the explaination that was in this scene in the WN.
“When you increase the concentration of magic, it will become denser and have different properties.”
Olga-san started to teach me a bit more about these rules. He also taught me that as the color became darker, it would become harder to heal myself. I should take into consideration that demerit.
In fact, I noticed it too. I tried to heal myself with that denser healing magic but I wasn’t able to. Olga-san was naturally born with denser healing magic and even he had a lot of trouble when it came to healing himself. I could somewhat understand these results.
So if the idea behind healing wounds is assisting in the body's self repair, then maybe the idea behind curing diseases could be assisting the body in destroying pathogens. At least with most diseases that are caused by pathogens.
This fic at least made a good reason why healing magic isn't highly valued and why people naturally inclined for healing magic are treated as treasure. I just...can't quite completely remember the details. Wasn't it that healing magic can technically be used by anyone but is difficult to use and you need an extremely high degree of skill to use it at any level that's actually useful? And thereby making it pretty much pointless unless you have a natural talent for it?
the way I see it is he didn't hurt his hand condensing "healing" magic, they were looking at the normal colour their magic took so they were condensing just "magic" so his exploded when it reached critical mass.