I don't really see anything wrong with it. The people using life pods to escape were doing exactly that, trying to escape as opposed to abandoning a sinking ship as the ship was in no danger of destuction. Its no different than straffing a retreating column of infantry.
The Empire obviously did take prisoners other than Leia from the ship, I guess they were all executed for treason like she was about to be?
The moral dissonance in your post really kind of blows my mind. So there's nothing wrong with blasting troops who fled in unarmed escape pods to avoid a certain execution (and in all probability, torture and interrogation with a mind probe)?
If the Empire wished to conduct this operation in an ethical manner, then they should have, you know, not executed the surrendering prisoners (IIRC the EU confirms no non-droid survivors except for Leia) and they could easily have tractored escaping pods instead of blasting them. But then, if the Empire did very many things in an ethical manner, the Rebels wouldn't have been out there in the first place.
According to the EU the
Tantive IV was in fact shortly destroyed, so it's fair to say the ship was in imminent danger of destruction as well. Again, this is why the Rebels were out there in an armed ship...
the Imperials are dicks. (At the very least the ones who are taking direct marching orders from Vader at the time.)
EDIT: Another thought just crossed my mind. Remember the first line of the movie?
C-3PO: "Did you hear that? They've shut down the main reactor. We'll be destroyed for sure! This is madness!"
It sounds like a probable sequence of events is that the captain of the ship attempted to surrender and complied with Imperial orders to shut down his reactor. Of course, it's also possible that the reactor was about to blow or something, but consider the following:
1) C-3PO,
of all people, thinks shutting down the reactor is a
bad idea and will lead to "being destroyed" by the Empire. He turns out to be absolutely right.
2) The captain, in his last oxygen-deprived moments, is lying to Vader in order to buy time for Leia, but he's sticking to the "we are on a diplomatic mission" line. If he opened fire first on the Star Destroyer, Vader wouldn't need to nitpick that the ship had no ambassador. This is evidence that the
Devastator simply popped out of hyperspace and blasted the
Tantive IV, leading to a frantic attempt by the command crew to save the ship and mission, first by returning fire, then by submitting to boarding. Leia's guards put up armed resistance, but a) it was justified considering the stormtroopers not only BLEW UP THE DOOR, but FIRED FIRST, and b) they may not have even known that the ship surrendered since everything happened so fast. A number of them did surrender in the end and look where that got them.
So yeah, in this context, blowing up escape pods might not have been EXCEPTIONALLY atrocious. It was just the cherry on the atrocity pie.