1. if they're so powerful, how were they captured?
2. why not just kill them? that's the question with just about every prisoner on belsavis
1. Simple, they fell into the old cliche of thinking their power made them invincible,they got overconfident and got beaten and then captured.
2. If you played KOTOR at all you'd know the Jedi do not kill their prisoners. Instead they imprison them or screw with their minds a la Revan. What they should have done at the very least was separate them in Belsavis, as they are more powerful together. The Republic had to learn the hard way to at least put them in stasis after they killed every living thing within a mile radius in their prison until the Republic smartened up enough and put them to sleep.