I accidentally killed him, but he won't reappear! Help,,,,, --- Humankind cannot gain anything without first giving something in return. To obtain, something of equal value must be lost.-Alphonse Elric
He will only reappear if you do a soft reset (unless you were stupid enough to save after fainting it). I think. --- Experience beats bias, but words on the Internet are meaningless. 3DS XL FC: 2492-4470-1418 / White 2 FC: 0047-4149-5185 / White FC: 0348-0626-6780
Didn't you save before battling it? Golden rule when it comes to legendaries :P If you saved afterwards, the only thing you could try is beating the Elite 4 again, I believe some legendaries would respawn in previous gens though I might be mistaken.
But yeah, you should definitely save before trying to catch a legendary mon~