I wonder if we'll ever know why Hakuhi did that? I guess Kanki is also needlessly cruel, but to tie people up and then bury them alive just seems... Time consuming. Wouldn't beheading them be quicker, if the goal was simply to eliminate 400k people? It must have been to instill terror in Zhao and get them to capitulate, but even so. Hakuhi was a real sadist. Especially since there was probably no need to kill them anyway.
But, that's the sort of thing Sei and Shin want to change. Conquering without destroying the local populace.