Jamieson, Fausset and Brown on 1 Samuel 6:19

19. he smote the men of Beth-shemesh, because they had looked into the ark--In the ecstasy of delight at seeing the return of the ark, the Beth-shemesh reapers pried into it beneath the wagon cover; and instead of covering it up again, as a sacred utensil, they let it remain exposed to common inspection, wishing it to be seen, in order that all might enjoy the triumph of seeing the votive offerings presented to it, and gratify curiosity with the sight of the sacred shrine. This was the offense of those Israelites (Levites, as well as common people), who had treated the ark with less reverence than the Philistines themselves.

he smote of the people fifty thousand and threescore and ten men--Beth-shemesh being only a village, this translation must be erroneous, and should be, "he smote fifty out of a thousand," being only fourteen hundred in all who indulged this curiosity. God, instead of decimating, according to an ancient usage, slew only a twentieth part; that is, according to Josephus, seventy out of fourteen hundred (see Nu 4:18-22).