This applies to Legendary difficulty, I don't know about other difficulties but I imagine it's the same. I haven't seen this information anywhere online, there seems to be some confusion about when Attila will stay dead, but I can confirm that I killed Attila multiple multiple times, and it never triggered any events until Chapter 4. I actually managed to kill him on the turn that Chapter 4 began, so I can say this with some confidence. I defeated his army once, and an event called "Only a man" came up, and when I followed his routed army up he died for good, which stopped the special Hunnic armies from spawning. Furthermore, I believe that if Attila is not killed in Chapter 4, he is scripted to die of natural causes at the beginning of Chapter 5. Hopefully someone can get some use out of this post, as the way to kill Attila permanently isn't very well documented in game.