I think what's weird about this is that it's being used as a motivational/interesting factor in the question. Something to 'make you want to complete the whole problem to get the answer'.
The difference with the trebuchets is that studying medieval people killing each other IS interesting and it's a past event. Anyways that's mostly done in physics, not in geometry. In calculus I think we one time had a question about a cannon ball but that was it. It was completely non-violent.
In physics we had trebuchet like questions or arrow questions all the time but they never involved killing people.

However if they did I could see it being more along the lines of historical than anything.
In this case we're talking about the 'potential future' (however unlikely it is). Not only just 'the potential future' but that of the President of USA... who gets by far the most amount of death threats than any other president (AFAIK). He's a highly public figure and people will be quick to draw the seriousness of a 'plot' to kill him... vastly different than 'Johnny has a gun high should Johnny aim to hit Bob in his head if ...'. Compare that to 'Johnny has a gun, at what angle should Johnny shoot to hit the Pope in the head if ... ' Even if the teacher used real students names in his class (mine used to do this all the time) it would be seen as comical... there's no 'realness' factor to it because these students most likely wouldn'tw ant to assassinate one another.
I think that this teacher should be more than just suspended, I'm not sure what though. I mean I don't think he should be arrested I highly doubt he intends or had intended to make a threat to the president. Maybe he should lose his position though? Even that sounds too harsh.