By law we cannot deport citizens no matter how undesirable. But it is insane to import non citizens to help them. Khalil is not a U.S. citizen and his behavior shows him as not the sort of people we want as U.S. citizens.
The President has no power to remove corrupt citizens from the country. But he can certainly approve ICE and the CBP removing non citizens who present national security threats or otherwise are undesirable as citizens. Instigating campus unrest to the point of unlawfulness, encouraging violating the rights of Jewish citizens, supporting terrorist groups is not what we want of citizens. It goes far beyond any standard protection of free speech under the Constitution.
Edit: I am NOT anti immigrant at all. I have done hands on work helping new immigrant families assimilate into U.S. society. I have taught Constitution to new immigrants (plus a few citizens who asked to be included.) I have worked with, befriended, been neighbors with a number of immigrants. I am all for new immigrants who come here to be Americans, who respect our laws, our values, our flag, our Constitution, our people. They can be Democrat, Republican, apolitical or whatever and it doesn't matter. If they become good citizens they are welcome.
But to allow immigrants, not yet naturalized, to stay here who disrespect our laws, our values, our flag, our Constitution, our people and who pose a national security threat by defending and supporting terrorist groups. . .
That's just nuts.