I find it very interesting to note that in PoD, there were three deaths that may be related to one another Ispan, Falion, and Jaichim Carridin. All three of these darkfriends had something in common. They were each attempting to locate the cache of ter'angreal secreted in Ebou Dar. Ispan and Falion were searching under orders from Moghedien (who may or may not have been under orders from Moridin - unfortunately I don't have CoS available right now to check) and Carridin was searching under orders from Sammael. Since the cache was located and successfully smuggled out of Ebou Dar by Elayne & company, these characters have outlived there usefulness (at least that's the way it seemed to me).

I think Carridin is executed because he is a servant of Sammael's. Regardless of whether Sammael is alive or dead (Alive!!!) he is not in the DO's best graces for trying to kill Rand and as a result, his servants suffer.

We find out point blank that Falion is executed because she failed in her assignment.

Because these two were executed under orders from Moridin and they both had the same assignment, I find myself wondering if whoever killed Ispan might have also been under orders from Moridin. As to who killed Ispan, your guess is as good as mine.