Sforno on Numbers 16:3/ספורנו על במדבר ט״ז:ג׳

1ויקהלו על משה ועל אהרן, Korach, Datan and Aviram, assembled in order to incite against Moses and Aaron in the presence of 250 dignitaries of the people who had allowed themselves to be incited. This incident occurred at a time when a number of Israelites had assembled in the vicinity of Moses waiting to have their respective complaints adjudged. The 250 men mentioned walked around as if minding their own business, wanting to consult Moses about some harmless matter. It was their intention to arouse the crowd in order to provide support for Korach’s and his henchmen’s insurrection as soon as the latter would show up and begin asking hostile questions of Moses and Aaron. They selected an hour of day when there was a sizable crowd in Moses’ vicinity in order to reap the maximum benefit from that, and so that all those who were convinced of the validity of Korach’s complaints would spread the word and bring reinforcements from the plain people in the camp.

2כי כל העדה, every one of the people

3כולם קדושים, from head to toe; (compare Numbers 15,8 you will be completely holy”)

4?ומדוע תתנשאו; in respect of the sanctity, by forbidding the firstborn to carry out sacrificial service in the Tabernacle? Moses himself had performed priestly duties during the first seven days of the consecration of the Tabernacle whereas subsequently Aaron and his sons became priests on a permanent basis.