Complex sentence: Dorr insists that he is stupid and tells him that he has to do the killing, for which Lump tries to shoot Dorr instead because he "doesn't want to harm a nice old lady".
Split into simpler sentences: Dorr insists that he is stupid and tells him that he has to do the killing. Lump refuses and tries to shoot Dorr instead because he doesn't "want to harm a nice old lady".