Angron is bound to end up working with us
He can run away for a millenia if he wishes too but chaos corrupt and impregnates everythinf so if he want to be free he will need to face chaos and the only way to contain chaos is by cooperating with pandora
Plus she is the only one that gives a single fuck about his life so unless he wants to be pursued by crusades of fanatics untill death he shoulf side with her
He could also just mooch off her domain indefinitely or go off to the Halo Stars where they did not even hear of the IoM or the Primarchs. He does not have many good options, but he does have options. That was rather the point of freeing him.