Yes, Moridin was very open and honest with Rand. He was also being just as manipulative as ever. In particular, Moridin wants existance to end, he wants the DO to break he wheel and unravel the pattern. There is no better way to aide the DO in this than encourage Rand to go around balefiring everyone. Moridin let Rand know that the only way to kill forsaken for good is with balefire so that Rand will use balefire to help things along - and further reduce his humanity.

It also strengthened the connection that eventually allowed Rand to channel the TP - that single moment was one of the shadow's greatest victories. Rand became so hard that the DO was able to touch him directly - something like Aridhol, but at the same time the very opposite of Aridhol. Rand was so dark that his ta'vern influence become completely bad - food spoiled, people died, etc.

So, while I'm not saying that Moridin wasn't honest here, but he was still very manipulating and Rand fell for it all...hook, line and sinker.