The middle finger is considered bad because historically and culturally it is a gesture symbolizing disrespect, contempt, or insult. Its origins date back to ancient Greece, where it was used as a phallic symbol meant to provoke or insult others. Over centuries, especially in Western cultures, the gesture has come to be widely recognized as an obscene hand sign equivalent to saying offensive phrases like "fuck you." Although it originally referenced the phallus, today it is generally understood as a rude and offensive gesture used to express anger or frustration toward someone.

Interestingly, research has also suggested that making the middle-finger gesture can temporarily reduce pain, possibly because it acts as a form of emotional release or distraction from pain, similar to swearing. But culturally and socially, it remains a symbol of disrespect in many parts of the world.

In summary, the middle finger is "bad" because it conveys strong negative emotions and societal disrespect rooted in its ancient symbolic meaning and continued modern usage as a vulgar insult.