The speaker has run away from home, and the poem follows them in the moment of leaving. Their parents are out searching, sirens sound in the distance, and the speaker refuses to look back. Instead of feeling guilty, they grin at the "pitiful attempt at care" and say they don't give a damn even if they're told they are loved.
Underneath that hard tone, though, there is hurt. The speaker believes that when things go dark, people show their "true feeling," and they distrust anyone who claims to care. They also feel they can't tell anyone what is wrong, and they ask why others need to see them "weaken." That suggests pride is covering up pain.
The mood is cold, defensive and lonely. The short, clipped lines feel like hurried steps, and the closing image, that in darkness "even your shadow leaves you," leaves a sense of total isolation.