In the primary half of the game, Zant was a remarkable villain. When he first strode into the throne room of Hyrule Castle, we knew he stood for trouble. And then he halted a few feet from Zelda and nonchalantly announced: "It is time to choose: Life or Death. Oh yes... A question for all of Hyrule." His composure sent a chill down my spine; if he could cockily invade the highest place of Hyrule without taking one casualty, then he might even cause those with the most rigorous courage to shudder. Zant bore this same unflappable aplomb when he confronted Midna and Link in Lanayru's chamber. He made Link revert back to his beast form and exposed Minda to the light she 'cherished so much'. So in addition to being imperturbable, he was cruel to those who resisted and or refused him.
But in the latterly half of the game, when Link finally infiltrates the Palace of Twilight and fights his way to Zant's courtroom, we find Zant laxing back on his throne. Almost immediately there's something noticeably different about his behavior. He starts out speaking calmly as ever, but as soon as his mask comes off he becomes a raging psychopath. He warps around various spots in the room and even dances around some, as if performing a melodrama while recounting his plan. At that point all credibility of his former character dropped for me. It made me wonder, Was he always this crazy? I'll admit his daftness somehow made the fight more appealing, but I'd still rather see him maintain his confident nature.
Since there was no smooth transition from collected to crazy, as Myriadviper42 mentioned, this brings rise to the belief that Zant could have been controlled by Ganondorf. That's a bit of a stretch, I'll admit, yet when Zant told the tale of how he'd formed a covenant with his master, Ganon said something along the lines of, "I will give you my power..." Desperate as he was from being cast out from the Palace, Zant accepted; but his connection with Ganondorf must have caused his mind to go unstable, and slowly lost its normal functionality. The way I see it, when Ganondorf removed his support from Zant when Link made it into the real Twilight Realm to unseat Zant. And so he was reduced to nothing more than a pawn which Ganon exploited for his own devices.
Zant was an awesome villain to begin with, and even when he turned loopy he wasn't so bad. I just think it would have been better if Nintendo let him be the ultimate antagonist, although the Twilight Princess rendition of Ganondorf was/is my favorite. Therefore I'm kind of torn between them.