Rand wouldn't join the Shadow, despite all the rewards Ishamael tried to lure him with in the early series. That's a dead-end, Rand like LTT before him obviously didn't and still doesn't care for Shai'tan's rewards and Moridin is not offering them anymore.

Now, Moridin was simply telling Rand the "plain" truth (as he believes it anyway - we don't know for certain that Shai'tan isn't lying to Moridin about "the end of everything", because it's what Moridin wants, the souls may rather end up as Shai'tan's playthings for eternity for all we know, trading the abstract Wheel for a very concrete slave master): there are no reward, the reward is "the final death" for everyone and for reality itself, the end of all suffering, "eternal peace". For a desperate man without hope left and overwhelmed by frustration and pain (emotional and physical), death can come to be seen as peace, but LTT/Rand knew all too well this wasn't the end for him, that it would start all over again, which Moridin made sure to remind of again, as he reminded him of Ilyena and being there when LTT died.. and here he was again, no longer Ishamael but like LTT being "reborn" as someone new.. who is still the same, caught in the same war. What's left to Rand not to go through that again without joining Shai'tan? Destroy everything himself. Or "kill Shai'tan" (we know why Moridin thought this a stupid notion: the amount of energy necessary to destroy Shai'tan would actually destroy Creation as well. For Moridin himself the result would be the same: final death, but I doubt Shai'tan would agree that being destroyed too is an acceptable bargain to destroy Creation...)

Rand won't join the Shadow willingly - Ishamael has known this since Falme, but there's still hope of making him want to give up on Creation and bring him to destroy it himself (using the Choedan Kal). Moridin wanted Rand to lose all hope, to be overwhelmed by frustration, pain, grief, madness. That's why he revealed to Rand that all his efforts, all the deaths and sacrifices against the Chosen since TDR had been in vain, adding to Rand's pile of things that have gone wrong since EOTW - he had to start his fight all over again. All this pain and most of the Chosen still live. And Moridin told him how to make their death permanent, warning of the cost: if he went that way and succeeded, if Rand wins, it will have to start all over once more, and again, and again... the battle will never be over unless Shai'tan wins. Rand can suffer through countless more lives.. or end it now.

Moridin has altered his tactics a little - shifting from "your cause is hopeless, join us and you'll be rewarded" to "End it now or we'll both go through this all over again, the same meaninglessness, the same pain and suffering for nothing, because there's only one way this can be over for good", but his strategy is essentially the same: Betrayer of hope he still is.

Moridin told Rand about balefire preventing transmigration simp;y as a as part of his larger efforts to further intoxicate Rand with his nihilistic worldview. Moridin was aware that Rand met frustration with his plans in Arad Doman (on his own orders), the timing was good to try to push him over the edge. Bringing Rand to use balefire at large with the CK (Rand came very close, twice in TGS), to teach it to his followers to kill the Chosen (and that he did), was another step in that direction, a sign Rand stopped to care or at least stopped seing the fine line he can't cross because the means he uses can just as easily bring about the Shadow's victory in another way. The Light had stopped using balefire against the Shadow in the WOS, realizing that balefire could bring about the very same result as letting Shai'tan win. The Forsaken had stopped using it as a mass weapon: they want their rewards of eternal life and power, not what Ishamael claimed would be the result of Shai'tan's victory (he's mad, they say).

It sounds to me like having Rand destroy Creation as a kind of global and final suicide or by accident/madness is seen as the "best option" for Shai'tan and Moridin, whereas fighting the Last Battle and having Shai'tan attempt to destroy reality remains "plan B" - much more difficult to accomplish, with a lot more uncertainties and obstacles in sight. We don't know that the Forsaken have figured out how to help free Shai'tan, or if they expect that once the sealing is gone Shai'tan will at last be ablee to free himself. That was the wall the Shadow hit in the War of Power : in over a century of Collapse and ten years of full scale war that the Shadow was finally about to win, Shai'tan still had not managed to fully free himself. If Moridin and Shai'tan suspect they'll hit the very same wall again, then having the Light destroy Creation remains their best chance to win.

I highly suspect that Rand and the two ta'veren are what hold the Pattern together at this point, which probably would make it still fairly difficult to unravel with massive balefire, in part because it would mean Shai'tan would have to drop the mask about the promises of eternal life and a world to rule over, an impossibility as the Chosen's instincts of self-preservations and their selfishness are so strong and the last thing Shai'tan would want is for the Chosen to realize where this is all heading and turn on him. Moridin can say it openly, Shai'tan can't and won't).

It sounds like killing Rand isn't a decision Moridin/Shai'tan would take lightly. Killing Rand might perhaps lead to a draw, a long war, the failure of prophecies, an empty world... with Shai'tan still unable to unravel it. It might mean at best a draw. Keeping Rand alive is a gamble, but offers a chance for full victory.

But it's not something the Chosen can be told about openly, Moridin aside. Indeed, they got strictly no explanation why Rand must live until Shai'tan says otherwise, just the order that it is so.

Rand's epiphany on DM is the turning point: at that point Moridin/Shai'tan had nearly succeeded, but instead of falling over the cliff they had pushed him on the edge of, Rand has hit the bottom of the barel and rebounded, destroying the Shadow's best hope with the CK's destruction. It's doubtful Rand has anymore the mean to destroy Creation himself, and now shattering him will be more difficult than ever as he's ready to settle for rebirth for himself and those he loves, for being the Dragon again so the cycle goes on. There's no angle to work anymore. It sounds to me like the Shadow's strategy will probably shift completely as soon as they realize this happened. The War will happen and Rand is no longer of any use. From this point on, he needs to die or his soul to pass to Shai'tan's control one way or another. Shai'tan might also be more cautious about expending the Chosen since using Rand to win is no longer in the cards. Of course, there's the possibility the Moridin and Shai'tan don't realize this has happened and keep going with their current strategy to "shatter Rand", not realizing that with each blow they rather strengthen Rand's resolve to go on and win from now on. They counted on Rand losing himself, but this rather brought him to the shattering point where he found himself again.

Shai'tan and Moridin played a daring strategy that could have paid off but in TGS they lost, losing Graendal and Semirhage in the process (not huge losses, but Shai'tan is running out of Chosen to lead in a Battle that might need more Chosen to lead than they hoped) , and giving Rand a mean with which to remove the others for good. Another "big loss" is that after his "epiphany" it's most unlikely Rand would manage to put himself in the "cold void" in which he could embrace the True Power, so that option is also no longer on the table, meaning when Shai'tan apparently on a whim gave up on Semirhage bringing Rand to him for the "bonus" of giving him a taste of the TP, he made a mistake in the long run. The Shadow will need to fight and win the hard way now.