He funded the Naryshkin Institute to research black holes, hoping they would find a solution to his situation before it caused the Earth's destruction. He could slow down his conversion by absorbing the life force of others, compressing the bodies to small pebbles.
When George Williamson was freed from his icy imprisonment, his travels created a time envelope extending into the past. Sabbath extended it to before Event One, convincing Curtis to travel all the way along it. He said the release of the energy within Curtis before the universe's creation would be safe.
However, the Eighth Doctor realised it would have catastrophic consequences. He convinced Williamson to travel back to 1894 and cause his death definitively. Williamson did so, and his ghostly image startled his younger self so he did not seek shelter from the explosion of a grenade. Williamson's death meant he did not travel in time and create the time envelope. Curtis, already in the envelope, was killed as it collapsed, the energy within him safely distributed across time. (PROSE: Time Zero)