The sack is long, dark, and torturous to go through, like Ivan's life had been. At the bottom is an "escape," which Ivan is afraid of even if he desires it. But, at the end of the sack isn't really total death, just the death of Ivan's false life. The death of Ivan's false life means a new beginning, though, Ivan has to stop clinging to his false.
You can see the black sack as a symbol of Ivan's sickness. It's painful to go through his illness. and escape is death. Bodily death certainly would put an end to Ivan's illness, and it's obvious why he would want it even if he's afraid of it.
