Kuma's crimes before being tamed by Koko made his death expected. That's karma.
I do hope Koko isn't dead, or negotiations will be permanently off the table and Yuuki's sister will surely die down the line.
Imagine if her power to grow wasn't immune to the law of physics. She'd have her bones crushed and flesh slurried if she tried to expand somewhere that was stronger than the force that makes her grow.