Remember he had drugged her though, so her faculties were reduced.
That raises a question from the opposite side. Why would a sluggish, drugged Natalee be curious enough about what was on Joran’s laptop to go poking around on it?
10 gets you 100 that the Peruvian goons told Joran he could get off easier on a confession to a “crime of passion” (this holds true for a number of Romance and Latin American countries whose law is based on the Napoleonic Code). Rather than the cold, soulless thing it would have been seen as.