He is a potential shooter Grant is supposed to be arresting, but he takes on the "red flag" case and doesn't arrest him. Instead the rest of his team 'takes care of him,' preventing him from going on a shooting rampage.
There are three potential reasons the core team does this.
1.) Grant was meant to arrest him and prevent the shooting.
2.) They needed a reason for being in the warehouse.
3.) Grant was originally supposed to confront him in the warhouse before the Travelers changed the Timeline.
Info to be added.