It is understandable that Moshe Rabenu had to bring a sign to prove that he was not making anything up on his own and everything was done as a mission from Hashem. We can understand why Korach and his men had to be punished in a miraculous way in order to prove this. But why did they need to be swallowed alive into the ground? Why did Moshe need to treat them so severely and cause them to suffer so much?
Rav Yehoshua of Belz zy’a answers that the opposite is true. Moshe was actually trying to help them. He knew that every Jew can always do teshuva until the last second of his life. If one does teshuva before he dies, he can be forgiven of all of his sins. But after he dies, he can no longer do teshuva at all. One second after death, all hope is lost.
He wanted Korach and his followers to go into the ground alive because he felt bad for them and wanted to give them a chance to do teshuva. If they had died right away, they would not have had the chance to see how wrong they were and he knew that they would not do teshuva. Therefore, he wanted them to be swallowed alive so that they would have more time to see that they were wrong and do teshuva.