Moridin wanted to stop, but not kill Rand. Mad channelers can cause more chaos than half sane ones. That's his motive.

Apart from Cyndane, they were forced to go. Demandred said the circle matched him in strength, so he could have stood and fight if he really wanted to be there.

Enough of the One Power to destroy the world was being channeled and killing the person doing the channeling could cause unexpected results. It’s odd that Cyndane and Dashiva didn’t think of that.

Lastly, they never work together. Same for the Light, but at least they are divided groups, where as the forsaken are just individuals.