Come To Think Of It ...

... according to the US Supreme Court ruling in Trump v. United States, Joe Biden could declare Donald Trump an unlawful enemy combatant, drone strike his ass at Mar-a-Lago, Trump Tower, or the Republican National Convention, and unless the US House of Representatives impeached him for it (likely) and the US Senate convicted him of it (unlikely), he'd legally be golden.

It would clearly be an "official act," and moreover one for which there are clear precedents (e.g. Anwar al-Awlaki).