I think their killer was Vandene, Adeleas' twin sister. Vandene was one of the few people that Adeleas would have trusted: clearly, Ispan's custody had been entrusted to both of them.

Secondly, Vandene is REMARKABLY calm following Adeleas' death: she slips in after Nynaeve et al discover the bodies, because "when [she] saw all of you heading this way, [she] thought..." (pretty weak...), and all she says is "What happened?" "coolly". Nor do we any grief or a crack in the "mask" Elayne thinks Vandene must be assuming.

Also, in 'A Quiet Place' it is Vandene who takes the lead in claiming Ispan. Although Elayne sees both sisters dragging Ispan away, it is Vandene who explains their actions to Elayne, she drags Elayne and Aviendha outside the makeshift cell and Vandene who suggests that her experience makes her a more appropriate candidate for questioner. "Vandene seemed to take the recommendation as accepted...". Adeleas does contribute to the conversation, but only to verify what Vandene says.

Thus Vandene is BA: she realizes she will have to get Ispan away from other potential questioners. Ispan must give Vandene away (clearly NOT Vandene's plan...), and Vandene must kill Ispan, and the other witness - Adeleas. Alternatively, Vandene may have been ordered to torment and execute Ispan as Shiaine was ordered to punish Falion later in the book. (Vandene has been a bit suss for a while now: remember the whole surreal episode with Moiraine and the dragkar in book 2).