I was killed instantly, and I can perform instant kill anytime, too. Maybe it was affected by mod, but I don't know which mod make my savegame strange. If anyone has any idea, please help me, I want to play Dawnguard

To be honest, it is very difficult to try to understand what your problem is, and whether you are trying to describe one problem or several.

Getting killed fast by a strong enemy or being able to kill fast a weak enemy can be typical to the game, and does not have to be a problem. And just saying that your savegame is strange, is not really telling anything.

Perhaps you could try to be more specific about your problem. Describe what exactly happens, and what exactly the problem is.