But it's a roll of the dice and what are the odds on killing Luigi triggering civil unrest that spirals out of control?

If they think the odds are low enough they'll do it.

The Americans should make it clear that killing Luigi will result in mass riots and the killing of more CEOs.

There are piles of dead martyrs, entire families with their own personal martyrs. Throwing another one on the pile is nothing to them, they’ve weathered it before. Him getting to make his case at trial and the chance he could successfully acquit himself creates far more unfamiliar and frightening possibilities. I am almost certain a trial is the worst possible outcome for those who feel threatened by Luigi’s actions.