lmao, the arm scene.
Although, the explanation that healing a healthy body causes necrosis seems really potentially serious. Saints/priests would be really OP then, with non-projectile magic that could just take a limb/decapitate, especially if they can do area heals/etc. If it was just self-inflicted damage from not being able to control a magic spell that'd be one thing, but the explanation was that it was the effect of the spell hitting an "improper target" vs just internal magic going haywire that'd only be capable of hurting the caster.
Plenty of magic systems where this is exactly what happens. "White Magic" used to harm, usually by the Bad Guys, because Obviously Evil Application.
Based on the theory that magic that is applied incorrectly
will find an effect/exit, generally in a way that is opposite the originally desired effect. Classically rebounding on the caster, but there are plenty of cases where the effect is deliberately directed at a target.
No doubt the correct, full spell has the condition that excess magical energy be dispersed harmlessly as part of the formula. Which the boy didn't get/missed... So he accidentally turned a High Heal into a Serious Harm without a stated target.
Bad. Move. Guaranteed Rebound...
As for weaponised use... Most people can't fling magic around like it's candy, and High Heals tend to be rather high rank/mana consuming. Area effects even more so. Generally more conventional application of stabbey-slicey stuff is more efficient, especially if you can use that mana to patch up your mates to do more of the slicey-stabby stuff. Efficiency matters..
Even if you can, doing so will most definitely Get You Talked About. And not in a positive way...
The principle of doing this is obviously known, as evidenced by our MC's reaction. Probably taught about in those Boring Lectures as well as part of Safety Precautions when mucking about with spells..
But using it in this way deliberately? Especially when
intent probably matters as part of the spell? Can't see many people using it that way deliberately. Unless, of course, psychopatic, Evil, etc.. applies.