The way I read the scene was that he was so high off his power that he thought with the tables turned and Pike having no power that he would have the upper hand. But Pike managed to subdue him and High Lord Zacarias was all of a sudden staring down the barrel of a phaser rifle and all of a sudden wasn’t so confident that the Pike he knew, who would lower the weapon once the combatant was subdued, was still there. I think the sudden switch was just an “OH shit, what have I done?” moment.