NeverarGreat said:
The Empire would have gone mad with power, destroying all planets suspected of harboring or aiding the rebellion.
Using this wickedly fun strategem ^ reaps ultimate defeat as proved by an extended playthrough of 'Star Wars: Supremacy aka Rebellion'. At first laying waste to whole systems instills universal fear but eventually there is a tipping point where every planet finally realises you've gone insane and cannot be reasoned with and so joins the Rebels :-D
NeverarGreat said:
But Tarkin was still trying to find the Rebel Base, so he would still have destroyed Alderaan...
Yes silly me you are right. But maybe it was the knowledge that two Droids were running around with the dangerous Death Star plans that tipped Tarkin over the edge, into commiting mass genocide. Without this time sensitive factor he might have just kept torturing/interrogating Leia for weeks until she broke, died or he gave up.