So what does it mean?
Rakanoth fails if he loses to the hero. Iskatus' fate was to be defeated by the hero.
So, what Diablo is saying essentially translates to:
"Don't get killed by the hero or you will get killed by the hero."
HURRDURRR! I ARE PRIME EVULZ!
Seriously, Chris Metzen, you're great at creating enganging settings, but please let someone else write villain dialogue.