Let me try again.
What assumption do you think should be made when someone disembarks from a plane leaving the checked luggage behind. when you answer this honestly we can move to step 2 in your logic lesson.
I did answer the question in context. If you are asking the question without any context, no assumptions can be made, certainly none that would justify shooting a passenger to death.
In context, Alpizar's wife identified him as her husband, and told the marshals that he was sick. In context, an appropriate assumption was that Alpizar was sick and needed to get off the plane.
And you can keep your "logic lesson," with your "logic" we would see hundreds of shootings in planes and airports annually; fortunately most air marshals are using at least some common sense, and not Raycpa "logic."
(and BTW, it's interesting that the pro-kill posters don't refer to Alpizar by name.)
Bag-matching is mandatory on international flights, and was proposed but rejected for domestic flights.
AFAIK, no "assumptions" are made - although a person deplaning usually gives a reason and is also interested in re-routing any checked luggage. Absent an explanation, the assumption is that the checked luggage (or carry-on items left behind) poses a threat to the aircraft.
In a situation of snap judgement, a shooter will not account for fellow travelers (unless they too are presenting an immediate threat), or think about much other than what is immediately apprehended as a deadly threat. In this case, the shooters justify their apprehension of a deadly threat by asserting that the guy made a credible bomb threat. They have added detail by asserting that the bomb threat consisted of words (running up and down the aisle yelling a threat), his carrying and control of a pack, the guy making threatening moves toward the marshals (after having exited the aircraft), the guy failing to obey orders, and the guy reaching to get a hand into the bag.
The checked and carry-on luggage was dealt with afterward, by putting it in a safe place and "detonating it." This is prudent follow-up, but is not likely part of the calculus that played in the shooters' minds in the moments that lead to their conclusion that the man himself posed a deadly threat.
The fellow travelers were dealt with as well, by assuming that any number of them could pose a deadly threat. Some of those travelers did not follow orders (for example, they guy who was peeking between the seats and talking on his cell phone); and those securing the plane were prepared to kill any perceived deadly threat. This too is prudent follow up, given an initial belief of a bomb threat.