Who or what is this greater evil Thrawn and the Witches are running from?
My guess:
Abaloth: The Mother of the Mortis Gods and the embodiment of chaos.
A ruiner of galaxies and a being whose sole desire is the entropy of all life the universe as well as it’s devolution into chaos.
She is also known to cause a mental condition called ‘Force Psychosis’ in force users, wherein they experience intermittent episodes of intense paranoia and fall under the delusion that everyone around them has been replaced with imposters (like in Capgras syndrome). The only other people that are recognised as real/authentic by afflicted individuals are others under the same delusion.
The reaction to this altered state usually results in the afflicted person attempting to kill ‘the imposters’ and the episode ending in returned mental clarity as well as full remembrance of one’s actions during that time, the episodes become more and more frequent until it becomes permanent.
It only takes encountering or witnessing her once to inflict the condition, with the potential to lay dormant for years and appearing randomly even decades later.
Which if this is true in the new canon and Abaloth is indeed this mysteriously terrifying force, makes me very worried for Ezra if he has become afflicted and Sabine merely caught him on a ‘good day’.
Previous theory expansion/connection: Thrawn and Ezra were at one point allies.
To tie it in; if my other theory that Thrawn and Ezra were at one point on good terms as allies during their exile, this may have been the cause for their separation. Thrawn believing Ezra was no longer stable or safe enough to return to their Galaxy, hence why he doesn’t want to risk Ezra being left behind alive as he could find a way to return with this being of certain death following right behind him.