Apparently, the term "brother's keeper" has a positive connotation because Cain used it derisively / disparagingly when attempting to deny having murdered his brother, Abel. But even if Cain used it in that way, it still actually has a negative connotation. A "keeper" is someone who guards and restricts the freedom of someone else - usually, an inferior such as an animal, as in the term "zookeeper."
Anyone using the term "brother's keeper" is actually insulting his brother.
Regards,