Betrayal is at the heart of "Broken." The speaker asks the same question three times: how can someone who helped put you back together be the one to tear you apart again? The repetition makes it feel like a thought that won't stop circling, as if the speaker has no answer and can't move on until they find one.
The verses in between explain why the hurt cuts so deep. This person knew the speaker "inside and out," saw them as "a work of art," stayed supportive even when the speaker's choices weren't wise, and understood their "monsters" yet still cared. That closeness is exactly what makes the betrayal painful. Someone who knows your weak spots is the one best able to hit them.
The mood is sad and bewildered, with a hint of anger underneath. The simple language and steady rhymes ("apart," "art," "smart," "heart") give it the feel of a heartfelt confession, raw and unpolished, much like the feeling it describes.