User blog comment:Xilinoc/559. THE NIGHT RIGHT/@comment-2211792-20131127151719/@comment-5398287-20131127193939

Y'know, I think I know why Yhwach chose to kill Cang and BG9 even though they were seemingly still able to fight.

He must have thought it an insult/disgrace for them to have been beaten by those with Hollow power, namely Hitsugaya and Sui-Feng. Kinda similar to why he killed Ebern even though he was perfectly able to fight.