Again, that might not be the case (and another freeper mentioned that he supposedly shot the hunters in their backs), but that is what the article reported as his statement.
And if you didn't read the pings to you on this and the other thread, some of the Vang threads in your link are about the mass killing Vang, not the killed Vang (there are two).
Relative to self defense, I think you were referring to Chai Vang, the convicted murderer. That claim came along very late, perhaps as late as his trial, and wasn't raised in his confession, where he indicated he was reacting to racial epithets. Other than the two or three hunters who initially confronted him for tresspassing, his other victims weren't armed and as I recall two were shot in the back. They had heard shots, and assuming a member of their party had taken a deer, hopped on atvs and went to the site unarmed, whereupon Vang opened fire on them. I believe there was a single shot fired by one of the initial armed victims. There were a two survivors, wounded, if I recall.