Okay, so bare with me here. At this point, we all assume Sayid didn't kill Desmond. Maybe it's our instinct, or maybe it's just the hope that the apparent Savior CAN'T die. The question of "what happens when someone dies" has popped up quite a bit. Whether people are referring to what happens to someone in the ALT after they die, or in the Original Time Line (OTL), it's the same curiousity. As we all know, when Juliet died in the premiere, Miles brought back her message to Sawyer, which was "it worked". Think about it. What worked? One thing that comes to mind immediately is the effort to reset things and make them normal. After all, that was the big plan. So let's assume that's what Juliet was referring to. This would mean that death in the OTL is the means of moving on to what OTL characters would consider a normal life. Couple that with the near-death experiences of Charlie (and Desmond to an exten! t) in the ALT, where they seemingly saw their life in the OTL. It kind of implies that death is the key between both timelines. And that by going through it, you move on to whichever universe you're didn't die in.
Now with that in mind, think of Desmond in the ALT. He's had somewhat of an epiphany recently, so much so that he seems to know exactly what to do to make things right. He knows he has to at least talk to his fellow 815 passengers. And lately, he's known where to find them, and what to say. Basically, he's CHANGED in the ALT. For the better it seems.
The question is why. And actually, it might be easier to answer than you think. As I said at the start, we assume Sayid hasn't killed Desmond. I don't think that's the case. I think Sayid followed through on his instructions to kill Desmond. After all, Sayid has "turned to the dark side". I doubt Des' little speech would have much, if any effect. So Des is dead. And by dying, he "awoke" in the ALT, knowing exactly what he had to do. This is why he's following through on his mission in the ALT and getting everyone he needs to see their would-be Island life. And this is why he's changed from the seemingly unaware version of himself in the premiere and a few episodes after, to the fully ambitious man he is now (all in the ALT of course). This means the ALT is slighty ahead of the OTL, from our perspective. Sayid killed Des in the past, not long ago, but in the past on the Island...and Des awoke in the ALT. We didn't witness the killing yet, but we have already seen the change i! n Des in the ALT.
As for the plan Des has in the ALT, I can't even imagine, but I think Des is indeed the one who will make everything right again...just not in the OTL. I think he willingly went with Sayid knowing full well he would have to die and change things in the other timeline to permanently change things in the OTL.