Liv didn't have the energy to argue that she wasn't a good person. She had infected Major, selfishly, in the hopes that maybe he'd see they could be together. When that severely backfired, she gave him the last of the cure. She was afraid to explain that to Bruce because she really needed to feel arms around her.
"I am disgusting..." she managed between sobs. She wasn't referring to the brain eating, her undead body which, who knew how long it was going to last in its current zombified state. Maybe she would eventually start to decompose, regardless of the number of brains she ate.
She was referring to her actions. To the reactions.
"I don't want to be this anymore..." she pleaded, even if she knew it was hopeless.