Is it obligatory for me to take back computer accessories that I bought and then saw being used for unlawful purposes? Does this include the computer's memory? And what should I do about the keyboard that I suspect I bought and my brother is using?

If tools are used for unlawful purposes, you must advise their users. If they do not respond, take them back if they are your property, as this falls under changing evil. However, if you have gifted them, it is not permissible to reclaim them. As for the painting you are unsure about having purchased, it is not permissible for you to take it, and you must ignore such whispers (waswas).