Why did the Emperor not help Angron?

Question

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?

Answer 1

The Emperor was a great strategist, scientist and psyker. But he was pretty poor grasp on human psychology and he was a really horrible father. In the "Master of Mankind" book, the Emperor is checking could he take the infamout Butcher's Nails out of Angron's head. When he finds that he couldn't (they are the only thing that keeps Angron alive), he is not even really disappointed:

Land hesitated. ‘You are more sanguine than I would have imagined in this moment, even knowing of your holy detachment from emotion.

‘What would the alternative be?’ The Emperor laid the bloodstained gloves on a nearby surgical trolley, where red-marked knives and other instruments lay wet and freshly used. ‘That I might mourn the Twelfth as though it were my injured son, and I its grieving father?[...] It is not my son, Arkhan. None of them are. They are warlords, generals, tools bred to serve a purpose. Just as the Legions were bred to serve a purpose.

‘The primarchs. It is said they have always called you father. It seems so... sentimental. I’ve never understood why you allow it.’

The Emperor was silent for some time. When He spoke, His eyes had returned to the hulking form on the surgical slab. ‘There was once a writer,’ he said, ‘a penner of children’s stories who told the tale of a wooden puppet that wished to be reborn as a human child. And this puppet, this automaton of painted, carved wood that sought to be a thing of flesh and blood and bone – do you know what it called its maker? What would such a creature call the creator that gave it shape and form and life?’ Father.

For the Emperor, Angron is just a tool. If the tool refuses to cooperate, you have to make him cooperate - cutting him off from his fellow gladiators was a way to do so. Without them, Angron had nothing to do with his life other than joining the legion. And it seems that for the Emperor it was inconceivable that any of his primarchs would ever go against him (yes, we had the two missing primarchs, but they were most likely rather "broken" than "rebellious"^*).

^{* Sanguinus mentioned that he was afraid to tell anyone about the genetic flaws within Blood Angels, because "they might be purged just like the missing legions". This suggests that there could have been some serious flaw in at least one of those two. Also, Roboute Guilliman declares "I was one of twenty. Two failed. Half the rest turned on my Father" this means that the missing guys rather weren't trying to make their own Heresy}

Answer 2

From the Lexicanum Article on Angron:

His fate seemed sealed when seven well-equipped armies surrounded Angron and his starving forces. In desperation, Angron fed his trusted comrades his own blood to keep them alive. Just as the battle was about to begin, the Emperor of Mankind's Fleet arrived in orbit over the planet. The Emperor teleported directly to Angron's point of deployment with a few trusted Adeptus Custodes. The Emperor promised Angron a legion made in his image, limitless power, and lifetimes spent perfecting the Art of Conquest. But, to his surprise, Angron refused. He chose instead to die amongst his comrades while fighting his oppressors. Reluctantly, the Emperor returned to his flagship above. Yet just as the battle was about to begin, the Emperor teleported Angron against his will back up to the fleet.

So, before teleporting Angron to the ship, he already had contact with him but Angron preferred to die with his fellow slaves. Angron could have easily asked the Emperor for help and would've probably got it. Or he could've asked the Emperor to take the freed slaves with him. But he preferred for them and himself to die.

I don't think that has ever been stated, but my explanation is that the Emperor saw that Angron was too bonded with those people and would stay with them while they were alive. So, the only way to get the Angron he needed was to let those people die and crush Angrons hope to free them. Plus, he probably underestimated the impact of the Butchers Nails on Angron.

Furthermore, don't forget that Angron was the only Primarch not to rule the planet they've landed on. So, something was clearly "wrong" with him and the Emperor had to make sure that he's not being held back as a General for his legion by the freed slaves.