From Sonny Bunch of the Washington post:
First off, let's dispense with the childish notion that Alderaan was, as rebel spy and intergalactic insurrectionist Princess Leia has argued, a purely civilian target. There is literally no reason to believe her claim that "Alderaan is peaceful, we have no weapons." She had previously lied about not only the diplomatic nature of the mission she was on when she was captured but also about the location of the stolen Death Star plans. It's also worth noting that she would go on to lie about the location of a military target for the Death Star to target moments before Alderaan was destroyed.
We have further reason to disbelieve Princess Liar when we consider that her adopted father, Bail Organa, was one of the original members of the rebellion, conferring with Obi-Wan Kenobi and Yoda in an effort to undermine the democratically elected Chancellor Palpatine shortly after his ascension. Alderaan was less likely a peaceful planet than a financial and intellectual hub of the rebellion, whose leaders, as we've seen, are totally untrustworthy negotiating partners — a hive of scum and villainy no less wretched than Mos Eisley, but on a planetary scale.
So, Alderaan was a legitimate military target. Was the level of force used against it justified? It's a tricky question, but it seems the least bad of all the alternatives. Consider another option the Empire could have taken: invading Alderaan, removing its leaders and installing a pro-Empire regime. However, putting boots on the ground in this manner would likely have destabilized not only the planet but also the entire region, creating a breeding ground for religious terrorists and draining blood and treasure for decades. It's not hard to imagine a Jedi State of the Alderaan System (JSAS, for short, though they'd likely prefer the simpler Jedi State (JS)) arising from the ashes of some ill-conceived invasion and occupation.
