Killing the Sheriff might satisfy Robin's "personal thirst for revenge", but it wouldn't improve the situation, especially since someone else might take up the Sheriff's role. Likewise, if Robin decided to align with the barons his group would have to accept amnesty, and how many of his "men" would feel good about this? There was no assurance that the barons' plan would work, plus this would require a real shift in overall mission from banditry to court political intrigue, and multiple stakeholders might object (how would the poor people feel?)
