The word "his" refers to Joab. "Blood" is a metonym for murder. And, the idiom "his blood on his own head" means the person should be considered guilty for murder. Alternate translation: "Joab has murdered people, and I want Yahweh to hold him guilty for what he has done" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-idiom]])
These words mean basically the same thing and emphasize that Abner and Amasa were much better men than Joab. (See: [[rc://en/ta/man/translate/figs-doublet]])