I believe China knows that N. Korea is a liability. The problem is right now, N. Korea is shooting missiles  because of sanctions. You cut off sanctions, more missiles would follow. Who's going to bear the brunt of that?

If you were held at gun point, and the cops on the speakerphone from the other side of town, still ask you to not give your money to the robber. What would you do?  Not a very good simile, i know, but pls understand the feelings behind it.