Dying cactus

The cactus in my toilet is dying. That is like so sad because I have no idea why its dying on me. After pondering for a while, I came up with 3 possibilities.

A) Someone pulled it out by the roots. (Cactus murderer! Argh!)
B) I added too much water.
C) I haven't watered it enough.

Sheesh. Who said cactuses are easy to handle?