Obviously the quick answer is, "She wents nuts in Shatterpoint. Duh."
But what about a few years later? We last see her in Republic custody, too far gone mentally to stand trial. It's incredibly unlikely that she'd get the death penalty as long as the Jedi are around and the Senate holds real power.
But that was before the dark times, before the Empire. . . .
Although there's only one Sith Apprentice, Emperor Palpatine proved time and again that he was willing to use Force-adept underlings (most notably Mara Jade) for various tasks. A homicidal former Jedi Master is, effectively, in his custody.
So did he turn Depa into an insane servant at some point, sending her (for example) on a mission that he deemed too suicidal for Mara Jade?
Y'know, for that matter, did he ever consider turning Mara into a full Sith and wacking Vader? Or would he have seen that as a step down in terms of raw power?