My response would be this: 1) The emperor had also convinced Ani that he was doing the right thing by joining him; remember Ani riding the lava and saying to Obi-Wan, "In my mind, the Jedi are evil." He had his mind twisted into thinking the Jedi were bad and the Emperor was good. 2) Ani was stuck at that point, there was no turning back. He knew he couldn't defeat the Emperor, and most of the Jedi had been destroyed. I'm sure after what he did to Mace (and trying to kill Obi-Wan), he didn't think he could waltz back in and rejoin the Jedi. 3) Ani had been turned into a power-loving Sith, and would rule the galaxy with the Emperor. He didn't have near that status with the Jedi, he was not even a Master.