I'd like to suggest a corollary to that: If something is bad when the villains do it, it's also bad when the heroes do it.
It's very easy to get into a situation where you have a double standard operating. After all, you know why the heroes are doing something questionable; they have all kinds of reasons, rationalizations and justifications. And the villains are just, well, bad.
The problem is that if you ask the villains why they're doing X, Y or Z, they'd come up with a ton of reasons, rationalizations and justifications, too. And having a double standard can irritate the reader. Unless you're writing wish-fulfillment fiction--and some people do--the odds are that the readers will be annoyed if you don't play fair.
So play fair. One of the best ways is to have consequences for an action and make them stick, no matter who they affect. If mind control is a bad thing in your universe and yet you really need to have your character use mind control to get out of a jam...fine. Let him use mind control. But let it come back to bite him in multiple ways. Maybe it weakened his magic. Maybe his magic is just as strong but now won't work properly. Maybe using a forbidden power has made him thirst for more taboo magic. Maybe he can't switch the mind control off now that he switched it on--you can imagine how well that works in everyday life. Maybe the people around him see his way of thinking and personality changing and start to lose faith in him.
And you can't run any of the film in reverse and make the consequences unhappen, because that's too easy.
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I'd like to suggest a corollary to that: If something is bad when the villains do it, it's also bad when the heroes do it.
It's very easy to get into a situation where you have a double standard operating. After all, you know why the heroes are doing something questionable; they have all kinds of reasons, rationalizations and justifications. And the villains are just, well, bad.
The problem is that if you ask the villains why they're doing X, Y or Z, they'd come up with a ton of reasons, rationalizations and justifications, too. And having a double standard can irritate the reader. Unless you're writing wish-fulfillment fiction--and some people do--the odds are that the readers will be annoyed if you don't play fair.
So play fair. One of the best ways is to have consequences for an action and make them stick, no matter who they affect. If mind control is a bad thing in your universe and yet you really need to have your character use mind control to get out of a jam...fine. Let him use mind control. But let it come back to bite him in multiple ways. Maybe it weakened his magic. Maybe his magic is just as strong but now won't work properly. Maybe using a forbidden power has made him thirst for more taboo magic. Maybe he can't switch the mind control off now that he switched it on--you can imagine how well that works in everyday life. Maybe the people around him see his way of thinking and personality changing and start to lose faith in him.
And you can't run any of the film in reverse and make the consequences unhappen, because that's too easy.