He was in favor of invoking the 25th Amendment against President Trump...but they are cute with their comments saying “he must be removed!!” which borders on what you alluded to...
Brennan has been a harsh and vocal critic of Trump, at times using severe political rhetoric—such as stating that Trump’s actions warranted removal from office via the 25th Amendment or writing that Trump would take his place in history as a “disgraced demagogue” —but he has never made any statements threatening physical harm or calling for violence against Trump.