He is. But we didn't always know how dangerous the Pillar was. Nico has power over earth and stone; he thought he could...communicate with it. Convince it to release some of the people who had gone into it before we arrived. So he and Aeris went out to try.
They made contact with something, and when they did, they both started...screaming, like they were in terror, or pain. The Pillar started to pull them into itself. Spock and I were there, and we pulled them out, but whatever they made contact with broke them. Shattered their minds completely. I tried for days to stabilize Nico, put him back together, but there was too much damage for me to reassemble on my own. We had to go into his heart, and destroy the echo of the thing, before he was whole again.