Well, use a auto clicker and a mouse recorder. Which basically will replay all of your mouse movements, and clicks if you record it once.
You'd press record, press respawn, do your kill thing, stop recording, and make the replay infinite. Then, just press play and it'll keep going. You'd hit multiple birds with 1 rock, but with an auto clicker, it only does half of what you need.
Though, there is no "auto respawn" mod specifically for Minecraft.