Let's say that hundreds of years from now our posthuman descendants finally do good on NASA's promises and build an Alcubierre drive or a true reactionless drive (either one works for this example). Great! Now we can traverse the stars! And if any uppity aliens show up and try to get fresh with us, we can just use our ship's drive as an infinitely powerful weapon with utterly arbitrary destructive potential to blow them, their planet, and even their solar system to bits if we wanted.
But would we want to? Fortunately or not so fortunately for our descendants, it's not too difficult to spot the major flaw in having an infinitely powerful weapon. You could never fire it, even in the vacuum of space, because all that energy has to go somewhere, and it may as well be the person, ship, or planet the weapon is being fired by (as well as everything else within a few light hours of you). This is absolutely unavoidable, unless the weapon is truly of a bizarre and exotic nature and doesn't inflict damage via thermal or kinetic energy, much like the hypometric weapons from Alastair Reynold's Revelation Space series.
So having said that, my next question is of course going to be "how do you avoid that?" Assuming you could weaponize a Q-thruster or the kind of Alcubierre drive that accumulates massive amounts of energy inside the warp bubble, would there be any way to protect the person(s) firing the weapon that isn't purely wild speculation (and by that I mean even more wildly speculative than Q-thrusters or warp drives)? I assume not, but I'm no physics expert.