This whole scene is very confused to me, let's try to recap:

• Elayne interrogates Chesmal
• unlikely coincidence: Sylvase's secretary happens to free Temaile and Eldrith just at the same time, and they meet into Chesmal's cell
• INCREDIBLE second coincidence: Mellar also happens to pass by just at the same time, apparently with orders to kill them

So... what would have been Mellar's plan, exactly?

To sneak by the shielders and the guards, and thus kill the Black Sisters while they were shielded? Even assuming he could do it with the first one, as soon as she died the shielders would have noticed and would catch him before he could kill the second one.
Or... to kill the guards and shielders (as Sylvase's secretary already did) and... and then what? If they were not shielded anymore, how could he possibly kill them?

Bottom line: Mellar was able to kill them only thanks to Elayne, and that certainly couldn't be part of his plan.