I don't have to. You can't guarantee that the hammer sold at DIY won't be used to bash someone's head in. The ability to use a tool for evil doesn't make the tool evil.
The purpose of a hammer--and here we'll go with a ball peen, claw, or sledge, not a warhammer--is to hammer in nails, hit chisels or punches, and break inanimate objects. It can
also be used to kill someone, but that's not its actual stated purpose. It's not built to be a weapon.
A mind-control spell's actual, stated purpose is to take over someone else's mind, thoughts, and emotions and deprive them of their free will. There is literally no other purpose for a mind-control spell. In fact, they literally
can't be used for any other purpose.
Do you understand the difference here?
And I consider taking over someone else's mind, thoughts, and emotions and depriving them of their free will to be an evil act.
Now yes, you can do it for good reasons--such as stopping someone from killing someone else--but you are using an evil action to do good. Maybe you feel that the ends always justify the means. Maybe in a case like this, the ends
do justify the means. But it's still evil means.
What does this have to do with my question?
Prison time for attempting to murder someone?
But how do you know they were actually going to kill the person?
Nope.
First, we're assuming that attempted murder is a crime in a fantasy world. Depending on the world or city, the guards and judges may not care what people in certain social classes do amongst themselves. Peasants can kill each other. Nobles can challenge each other to duels. Maybe it's perfectly normal for people to send assassins after their rivals, and if you get killed, it's your fault for not being alert enough and having good enough defenses. Nobody cares, except that you got in the way.
Or it could be that the victim was part of an underclass or race that could be killed with impunity. Imagine if the victim was a goblin or kobold citizen in a predominately human city. Hmm, would you even bother mind-controlling the attacker in this case?
And in a
realistic setting, the punishment might be death by hanging at dawn, in which case all you did was delay the inevitable by a few hours. Either that, or rotting away in a prison where the conditions are so terrible as to be considered torture by modern (i.e., player) standards.
As
@HammerMan said, drugging someone, coercion, blackmail, brainwashing, gaslighting, etc., are all either illegal or considered to be horribly immoral. All of these things can, technically, be used for "good" purposes--but that doesn't stop everyone from realizing that these are illegal or immoral.
So why should mind-control
magic be any different?
