She was nearly drained, too -- when people are hungry, they don't necessarily think as rationally as they would otherwise.
Lara is near a breaking point -- Thomas nearly dead, the whole city about to be plunged into Chaos, and a mortal world aware of vampires is one that will kill her.
She's not acting like she normally would, which is the point -- this is an extraordinary circumstance even by her standards, and everyone has a breaking point.