What is the ruling on taking my brother's computer headphones without his permission to listen with them, and am I sinful if he asks for them back?

Changing evil with one's hand is permissible for individuals who are legally capable of doing so, and it is not exclusive to those in authority. However, if changing the evil leads to a greater evil, or to the perpetrator shying away from the truth, then it is not permissible to change the evil with one's hand. In such cases, it is sufficient to offer advice and good admonition. Returning the aforementioned speaker to its owner is not considered cooperation in sin and aggression as long as it is his property.