I don't even think the directive was to kill all landlords (who were closer to minor nobles, anyway). I believe it was more along the lines of "let the people do what they want with them." In some areas this meant killing them (wonder what would make people want to do that?), in others it was punishment that was short of execution, in others it was redistribution of land but a fairly light touch otherwise.

"Fanshen" by William Hinton talks about this period some from an on the ground perspective, but I still haven't finished reading it. "Red Star Over China" might also.