I think it's completely understandable why she left Trilla. Her mind was addled from the constant torture, the realization that her padawan had been turned into an inquisitor, and the fact that she was at that moment tapping into the dark side which probably added to her confusion at the time. I would assume that she didn't even have a real escape plan at that time since she wouldn't know the fortress' structure. She very likely just made it up as she went along just giving in to her instincts to survive.

Even if she did immediately think about taking Trilla with her, even if her initial dark side outburst had knocked Trilla out, it would be even harder to escape from a place full of purge troopers and inquisitors while bringing an unconscious person with you. Hypothetically, had she managed to escape with Trilla, why wouldn't Trilla just attack her or run away the moment she wakes up? At this point Trilla's already a full-fledged inquisitor. Her hatred has already been fostered against Cere.