User blog comment:B214/Volume 23 discussion blog/@comment-2602:306:8075:390:ACFE:42D7:3070:DAE5-20170718181505/@comment-31137559-20170720204932

The reason why he used Chaos Break was because even if he defeated Kiba he still had Rias, Akeno, Koneko, and Ravel to contend with. And he knew he would be killed if he took  them on after fighting Kiba knowing that Kiba had improved to the point where Siegfried couldn't get off scot-free. So he used Chaos Break to fight Kiba and so he would be strong enough to take down the rest of them(although Ajuka could easily blast him into space dust). He never anticipated that the girls couldn't use their powers due to their sadness over Issei's loss. So you know how it goes. Completely one-sided match in favor of the enemy. Because of hubris doesn't kill Kiba right then and there. Good guy gets power-up. Bad guy is screwed. Bad guy gets defeated.

Do we really know the true reason why Gram left Siegfried and answered to Kiba? Saying it was because of Khaos Break sounded like an excuse to me. Maybe it viewed Kiba as its rightful possessor.