The Walking Dead has never been short on complicated relationships. After both becoming romantically involved with Abradham, Sasha and Rosita were not exactly friends. However, they bonded in Sunday's episode... and then Sasha seemingly sacrificed herself, locking Rosita out of the Savior compound and deciding to take them on herself. Will Sasha kill Negan on The Walking Dead? It certainly seems as if she's going to die trying.
I really liked their relationship in this episode. It's pretty clear that both of them were better as allies, and (not to speak ill of the dead, but) too good for old Abe. Did he really give Sasha a gift that Rosita made for him? Come on! Unsurprisingly, it took them a while to warm up to each other. Emotions aside, however, they make a good team. They amended the plan several times without argument, and even attempted to rescue Eugene.
Was it Rosita's survivalist skills that made Sasha decide to leave her behind? "They still need you," Sasha insisted. Maybe Sasha's death wish is stronger. Maybe she wanted Rosita to have another chance at life — one where she wasn't using people for their skills, as she admitted to doing with previous boyfriends in Sunday's episode. Even in the apocalypse, it's important to have your own identity. Carol is working on that as well.
Personally, I'm not too surprised that Sasha made a break for it. It's honestly possible that Sasha was preparing for this longer than we may know. She made Enid and Jesus promise to protect Maggie and the Hilltop? The Walking Dead presented a complicated, fascinating female relationship in Sunday's episode. It's a shame that, as is the nature of the show, both Rosita and Sasha will probably not survive to bond any further.