The problem is that even if he didn't have to be conscious to control Superman, eventually he's going to control Superman again somehow. It's not a matter of if, it's a matter of when. You can shove as many psi-dampeners in his brain as you want and throw him into the most negative of negative zones, it doesn't matter; so long as he's not dead, he's going to get out of it somehow. Period. Either he himself will find a way, or some other villain will learn about this and then bust him out. Think about it, a free ticket into making Superman do whatever it is you want him to do? Max would be like the Holy Grail for supervillains.
That's just the way it works and how it has always worked in these comic book universes, and it's partially why part of the League was against throwing Dr. Light into prison, too...because he's just going to get out again, or else some other villain will learn about how he did what he did and either help him do it again or try to do it him/herself.
Not a matter of if. A matter of when. Hell, remember the General from Morrison's JLA run? Superman and co. stuck him on single asteroid in the middle of frikkin' nowhere out in the vastness of space, and he found a way back. Kyle Rayner did the same thing to Major Force's head, sending it out into space, and now Major Force is back again as good as new.
Diana knew this and understood this with her wisdom, and that's why she elected to kill Max right then and there instead of risking that he would ever control Superman again, because the fact is that he will. And the next time he controls Superman, someone is gonna get their asses killed. A lot of someones, probably.
