A bit of spoilers but not that much. ( Dawnguard)
Heres what happen I just did the quest where you should save the moth priest and when I walked out from the dungeon he started to attack me but I made so he went down on hes knees and then he was friendy but that was not the end.
When I went back to the dawnguard he spawned and started to attack me and when hes on hes knees I can interact with him to start the quest and where ever I go he will spawn and start attack me after a loading screen plz someone help Even if I load one off my first saves when I just started he spawn.

Leave the Dawnstar valley/castle and wait for 3 days. That should reset his AI.

that could work

the Problem is that he follow me everywhere

Knock him out, and sprint away until he gives up and goes back to the castle.

I did not work but for some weird reason I got the quest after you lookin at him for 30mins XD but he still attacks everyone he have made a mess out of fort dawngaurd XD