sseamom gives a good example of a mob crossing the line and lynching blacks. A sad, real-life example of people going outside the law. It was a very real threat, that really did sometimes happen.
The threat, in this case wasn’t real.The first hurdle for the threat to be real is that, it was dependent on Trump’s winning the presidency. Like him or not, don’t we all agree his winning is no certainty? But if he’s elected, the high school boy feels Trump will enforce current immigration laws to deport those here illegally, including the Op’s daughter. Since we know she isn’t here illegally, but in fact is an American citizen, that is why enforcing current illegal immigrant laws against her are no real threat.
And no, a rapist does not have to rape more than once to be a rapist. But he would have to rape more than once to be a serial rapist. That is a part of the definition of serial rapist, committing repeated rapes. A number of links have been given here, including by carolinamom that define bullying as repeated behavior. One link did not, but the big majority did require it to be repeated behavior.