Final answer:
Humperdinck puts Westley to death because of a personal grudge and to prevent any further interference in his activities as a rustler.
Step-by-step explanation:
In the novel, 'Riders of the Purple Sage', Humperdinck puts Westley to death because of a personal grudge and to prevent any further interference in his activities as a rustler. Westley had killed Oldring, who was a prominent rustler and held Bess, a girl whom Humperdinck had feelings for, in bondage. Humperdinck sought revenge for Oldring's death and wanted to ensure that Bess was no longer a threat to his power.