Columbus, who had earlier tried in vain to convince John II to fund his expeditions, returned from his first voyage and made a visit to Lisbon before going to Spain. During this return, his boasting encouraged some of John II's advisors to advise the King to put Columbus in a position where he would overstep his bounds and run the risk of being killed in a duel. John II decided against it, for reasons of honour.
What if he didn't, and decided to off the impetuous admiral before he could send his letter or return in person to the monarchs of Spain? A suitable reason could be dreamed up relatively easily. How would this effect the return of the ships to Spain? What about Portugeuse-Spanish relations? Absent Columbus, would the Pinzons be more influential in follow-up voyages?