There's also ...

- having such an obviously terrible regime to complain about makes China look good
- doing anything about it would cost political, military and financial capital (you don't think Kim would go quietly, even if asked by China, do you?) that would be better deployed elsewhere
- related to that, if they tried to interfere and failed to fix NK, they'd look weak and the west could push them back in areas like the South China Sea, investments in Africa and S America etc
- NK is not really a problem for China, just stick a few border guards on the bridges (but NK do that for you already) and the cost of having NK on the border is small.