Wearing a MAGA hat and smirking
Being an ISIS terrorist
This is even more bizarre because from what I've read of the event there was a third group there going on a racist tirade, Phillips' group went up and confronted the students, and some of the students responded to that with tomahawk chop motions (making them nearly as bad as Atlanta Braves fans).
But the one kid who got singled out was not the one doing any of those things, and might have been telling the others students to stop. The entire reason for going after him is hating his face.
It's like if you had to kill one person from history, and instead of going with the obvious Hitler choice, you go with "the woman who posed for the Mona Lisa, because I hate her smirk."