Okay this is going to sound pretty weird but I have accidentally killed Alduin. I am aware you need to kill him but he has died a bit early, after reading the Elderscroll he came along all normal and everything and I kinda killed him there... Yeah I know it's not meant to happen. After he dies Paartha... whatever is still going "use dragonrend or else he'll kill you..." for a wise dragon he's a bit un-observing in current events. I looked through all of my mods and cannot find one that could possibly cause Alduin's downfall unless one of the texture ones had a magical extra that let me kill all. I have installed some kill all mods but they were unchecked so as my understanding of this goes, it couldn't have been them. Anyways thanks for any help you give

Did he explode?

Load an old save then when you get to this part of the MQ, use dragonrend then attack him but make sure you don't kill him.

You killed Alduin before the end of MQ? *Imitating WWE's The Miz* AWESOME!!!!!

How did you kill him????? Awesome !! How did you do it????

Did you ever enable one of the kill all mods on your current character and then disable it after while.
If yes then that is probably why he died.

If those mods use scripts, and they do, then they have baked themselves into your save. Unchecking them does not remove a mod's scripts from your save. Not even Uninstalling them (which is more thorough than unchecking) Rule of thumb with mods, the mix you have is the mix you keep, good or bad. Want to change your mix, load a save from before you added the mod you want to remove.

Which is why you should create a Test Character, preferably with no mods installed in the game (textures and race mods should be OK to leave alone) and use that for testing any new mod mix. Never save over this slot, and use the console to buff the stats if you need to for higher end mods/events. Once you know it's good, then load your character.

So, load a save from before you killed Alduin if you have had this mod for a while, and leave him alone. Until you are supposed to bother him.