The death of an American citizen outside our borders is out of our jurisdiction. So, in your example, the POTUS couldn't be charged with a felony. No US authority would have jurisdiction there (FBI, DC cops...nobody).

However, ordering a hit team (CIA, SEALs, etc) to take out a political rival in the US should be actionable, unless the SCOTUS rules that POTUS has immunity. Then we can expect a dictatorship to follow relatively quickly. We'll be no better than Russia, with Putin bumping off anyone he deems inconvenient.

To albaby's point, the POTUS should be second-guessing if he's that close to the line. Routine matters, such as diplomacy, military operations, etc, are one thing. Inciting riots and insurrection, or eliminating rivals, are not examples of routine matters.

I've occasionally mused that we should have a prosecutor investigating the POTUS during his tenure, and when he (or she) leaves office, if there is anything pursuable, we should have trials to see if they rise to the level of punishment. If he (or she) is "clean", they just retire. If not...