We all know that Angron, the Daemon Prince of Anger, was a gladiator on some far-flung planet (Nuceria?). He led a gladiatorial rebellion, with an apparent degree of success, taking many cities. However, when the Emperor came and asked him to join his campaign of galactic conquest and genocide, Angron refused, so the Emperor teleported him away. 

Thereafter, Angron's allies lost the battle and were all massacred. Later on, Angron's anger at the Emperor for this action would lead to him turning to side of Chaos. However, why did the Emperor choose to teleport Angron away, instead of helping him?

The Emperor has his own considerable psychic powers, his orbital superiority, undoubtedly a squad of Custodians, and maybe even some extra Space Marines with him, as well as the possibility to call for lots of backup. Given Angron's previous success, presumably this would tip the odds. Further, as we saw with Mortarian, he is not averse to killing tyrants on a Primarch's home world. Finally, all worlds have to be conquered anyway, so it is odd that he would want to wait. 

In short, why does the Emperor uncharacteristically teleport Angron away and flee instead of helping him win a battle that would only benefit the Emperor?