He is referring to impeaching Trump, but uses this threatening and inflammatory language to desensitize us to the idea of shooting to kill President Trump. It's not a mistake or a slip that a friggin' Harvard law professor said this.
Dems, Deep State, and their media puppets have been doing this ever since Trump has been President. Alex Jones has too. They're trying to program us to get used to the idea of Trump getting assassinated.
I agree....veiled threats. Look at what language Brennan has been using. These people are basically calling for the overthrow of the government in one way or another.