Potterholics: Bella vs Kreacher?

I think it is like in a case of divorce - both parties are to be blamed. Who is more responsible? Kreacher, in my opinion, because Bella was just in the right place at the right time to kill Sirius, and Kreacher actually helped to organize the Ministry battle. It could be Bella, Lucius, any other Death Eater in the Ministry that could kill Sirius.