When an actor CHOOSES to leave the show, it is their choice. Not the show's, as Heather noted. The show can write them out, or recast, if feasible. Sometimes they have to really scramble when illness forces an actor to depart suddenly, as in the cases of John, & Eve.
The writing for Abby, Chad has been horrible. Trying to stir up something using Dario & Gabi is abominable, the viewers have ranted everywhere about it.
As to the murder of Deimos...am guessing we get a few more "he/she did it", before the final resolution. As mentioned, it could have been Deimos's bodyguard, Xander, that waiter who also sipped at the Halo champagne......the amulet is gone, someone knew the value...and sure, anyone could have come in from outside, the kitchen, anywhere. Time will tell, but first we have to have all the "fake" suspects. LOL