In this parsha, Korach started to rebel against Moshe, and is joined by Datan and Aviram. They always were doing mischief for Moshe. They almost killed him – after Moshe killed the Egyptian who hurt a Jew, they told on him to Paroh (Pharaoh). Because of them, he had to stay out of Egypt as a fugitive.
Will Moshe get rid of them once and for all? No. Moshe sent out a peace delegation, even though he knew Hashem would take care of it. They responded in a disgusting manner, however. Imagine if you responded like that to the President of the United States!
Even though they responded like that, Moshe went to see them himself, with seven zekeinim (elders) in tow. He was trying to save their lives, but they didn’t listen and got swallowed by the earth, along with Korach.
Moshe is great because he decided to go do it himself, and not just let Hashem take care of it, even though he could have. He worked on himself, and if we do the same, we too can become great.