They were going to blow up Alderaan either way, you don't build a planet destroying super weapon and not publicly demonstrate that it works, then considering the Death Star was a weapon of fear rather than practicality it obviously had to be a populated planet, not just some backwater either, somewhere nice and well known.
It's like shooting the first hostage to show you mean business.
The empire was against that. Palpatine governed through fear and oppression, separating the people and the state. Just doing that is wrong, no matter if the people got their blue-milk by doing this. And don't forget - the old republic was functioning well until Palpatine used the trade-federation to create disorder.
And the empire could have functioned even better if things were given a chance to settle down, y'know there's nothing inherently wrong with living in a police state, if you're not a criminal then it can actually be quite cozy.
The very fact that the Jedi were needed by the republic demonstrates the difference, you don't need super solider negotiators if the senate actually a viable means of galactic management, sure independent planets and a galactic democracy sounds good but in practice that just means there's a lot of powerful people constantly trying to outmanoeuvre and back-stab each other. Without the Jedi order the Republic would have degenerated into a massive multifaceted civil war within weeks, and even with the Jedi there was still a lot of covert conflicts going on.
Whereas in the empire the use of fear enabled the imperial fleet to subdue an entire galaxy with a minimal use of force, granted blowing up Alderaan doesn't seem minimal but that was one planet for galactic peace, whereas if you read up on all the missions the Jedi went on it quickly becomes evident that beneath the shiny veneer the republic was rotten to the core, a powder keg the Jedi kept damp.
Quite simply the empire got by perfectly fine without the Jedi, indeed in spite of the remaining Jedi, and that if nothing else demonstrates that if the rebellion had been put down the entire galaxy would have been far better off.
A good dictator can be better than a democracy, it's true, a democracy is inherently corrupt, it only serves the majority and a democratic government can only do what the majority will allow it to do, whereas the emperor would have been solely interested in the power and wealth of his empire as a whole, because it's his empire, a reflection of himself and he wants a reflection he can be proud of.
