When using Cleave with Rupture do the kills I get from the exploding bodies count as a kill that will reward me with the bonus health per kill? Or does this only apply to enemies that were slain directly by my weapon through the cleave itself or otherwise.
Yes, you do as those monsters are actually dying from cleave. To add to your question Cleave exploding bodies count as as cleave itself. So this can trigger a domino affect as you kill 1 monster it explodes causing the one next to it to die, which explodes as well and so on, you would get life on death for each one of those kills.