My favorite what if that I cant help to think about the scenarios that could have played out.. Being one of the best fighters in the galaxy if Horus could have just avoided that one hit by the anathame. Could have even used a bolter to blast Temba to smithereens before he could get in close combat range. Seeing how Temba was clearly infected and mutated horribly if Horus would have been more clear headed going into the fight he would have kept his distance. How different the galaxy could be if he didn't let his need for revenge cloud his judgement in that one fight. I personally think if chaos failed to turn Horus it would have turned it's attention to one of the other Primarchs maybe Angron or Fulgrim. Many of the primarchs could have been swayed to choas just by they're jealousy at Horus being named warmaster.