I would think that if Trump was going to remove his own term limits so he could be President for Life and then start murdering his political rivals, it would have been in his first term when he had the House, Senate, and Supreme Court locked down. As it stands, he's only going to be in power for another four years, worst-case scenario. It would take a constitutional amendment to change that (which is a big part of why he isn't President for Life). I'm not going to sit here and say when it's okay to start killing politicians, other than that we aren't there yet.
I'd like to ask you a question as well: if Trump died, what do you think would have happened? Do you think that 100% of the gun-toting pro-Trump militias throughout the country would have laid down their arms and admitted defeat? Do you think that the political faction that is, on average, more likely to own and use guns than the left, would have said "well that sucks I guess"? Do you think that Democrats across the country would be safe? Or do you think it would be a Shot Heard Cross the Coasts that would have started a free-for-all of political violence that the country hasn't seen in decades - perhaps centuries?
While I do think that the right of the people to govern themselves has certain implications I won't get into here, it also means we have legislative options on the table. You have freedom of speech, which is why we can ask questions like yours and mine. We have the right to assemble, form parties, and elect officials. Let's use those rights while the government hasn't decided to destroy them yet; and if they ever do, let's take the discussion to a more anonymous forum like on Tor or I2P.