theoretically what would happen

[bilateralrope]

Presumably, this is the case for every weapon system regarding a safety. If you conceive of the transporter as a weapon, it could be just as easy to switch such a safety off. We are not talking about why the transporter isn't used as an impromtu weapon, but rather why it not a primary weapons system in its own right.

And while the Federation might have a moral reason to avoid it, there are plenty of transporter capable races out there that do not.

Building such an easy to use switch into your transporters is a tradeoff between the risks of the safeties being off (accident, hack, hostile infiltrator, etc) leading to friendly deaths vs the times when it would be useful.

Pick any season of Star Trek and tell me how many times in that season a weaponized transporter would have been useful. Unless you present evidence to the contrary, I will be assuming:
- If there is ship to ship combat, then the shields of either side will get in the way.
- If the ship is boarded in multiple locations, then assume that one of the off screen boarding parties was smart enough to neutralize the transporters.
- If the is any trouble tracking the threat with sensors, then you can't get a transporter lock.

[Patroklos]

Why does the transport transporter and the weapon transporter have to be the same system?

Pick any season of Star Trek and tell me how many times in that season a weaponized transporter would have been useful. Unless you present evidence to the contrary, I will be assuming:
- If there is ship to ship combat, then the shields of either side will get in the way.

There are many examples of transporter through shields, specifically the Dominion and the Borg. This is usually represented as a high tech or new not understood tech being encountered, but it also corresponds to primary morality light or morality nonexistent villains that should be using that advantage more effectively.

- If the ship is boarded in multiple locations, then assume that one of the off screen boarding parties was smart enough to neutralize the transporters.

This example proves the point. If they were able to successfully board that just means they have neutralized the defenders transporter weapon potential but just proved their own. If boarding in combat can happen at all, the transporter has been demonstrated as a weapon. Given these many instances of boarding of our hero ship were successful, but not follow on takeover, just this shows that the attacker should have been more creative in what they transported over (or out) of the target ship above and beyond boarders.

- If the is any trouble tracking the threat with sensors, then you can't get a transporter lock.

I am not going to pretend there is any point it discussing when, and when not, sensors work for any purpose in his franchise. We know they do in combat though often enough, even inconsistently. You might as well claim phases and torpedoes are useless given how often they fail in conjunction with required targeting (or otherwise prove ineffectual).

[Solauren]

O'brien beamed from the Enterprise to a Nebula class Starship (the Rutledge) using a window of a 60th of a second. Presumably, that would work in a combat situation if you could maintain a transporter lock.

[Solauren]

Which was a one-off weapon designed for use in situations where phasers wouldn't work that never went into production because they came up with phasers that would. Doesn't exactly sound like a very widespread idea.

You did see Odo scoff at that idea, right? And some random Vulcan pulled it out of the Federation databases, right?
It didn't go into widespread production, but still, it was a weaponized transporter. (Literally, it was part of the weapon)

[Patroklos]

The other thing to note about that episode was that at no point did anyone discuss simply blocking all transporters in order to prevent other murders. I doubt casual transporter convenience would have trumped preventing crew murders.