The US military has been fighting uniformed militaries in Iraq and Afghanistan for years now, and only ever call them "insurgents". Draw your own conclusions.
As with any group there are extreme or radical elements, the Rebel Alliance (which was backed by several rogue planets from the Empires point of view) was no exception.
Going through the lore you find that in most cases "insurgent" activity was limited to military or directly affiliated military support systems, with an emphasis on avoiding civilian casualties when possible. After all the whole point of the Rebel Insurgency or Old Republic Loyalists if you prefer was to win "Hears and Minds" away from the Galactic Empire. The use of terror tactics (making Imperial citizens afraid or intending to wipe them out) would not, in the end, be an effective method of recruitment or lead to the kind of government they wished to reestablish.
While on the other side the legitimate government of the Galactic Empire (as Jason Solo explains during the Second Galactic Civil war) made it a policy to use terror and fear of death/imprisonment/enslavement on whole populations as an institutionalized control method. The Death Star was exactly this, a weapon of terror.
In the words of Grand Moff Tarkin in response to General Tagge, "the regional governors now have direct control over their territories. Fear will keep the local systems in line. Fear of this battle station.”
The first act of terror was to annihilate an otherwise disarmed, non-miliatry, one would dare say pacifist planet of Alderaan all because a few of it's ruling royal elite were suspected financial backers to various insurgencies. As a point no trial was held or charges brought against anyone in the Alderaanian government and Princess Lia was held without trial in a mobile (and fortified) undisclosed location. The entire pollution and all the resources of the planet were annihilated simply because, again in the words of the Grand Moff Tarkin, "Dantooine is too remote to make an effective demonstration."
The Galactic Empire used institutional terror on its own innocent citizens as a way to dissuade any form of rebellion. From stormtrooper raids and the enslavement of a whole village to find 1 seditionist individual just for speaking out, up to the annihilation of a whole planet and population simply because a few of its leaders provide support to "terrorists". If the US was the Galactic Empire we would have nuked Saudi Arabia into a glassy radioactive pit because a small handful of it's elite financially support groups intent on bring the Western empire down. And groups with a preference for attacking civilian targets over military ones when possible.
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That's not to say the Rebel Alliance was totally clean organization. Many of their agents and affiliates used civilian populations as cover while striking at Imperial Military facilities. This of course prompted many acts of reprisal against innocent imperial citizens. In the early days of the Rebellion there was an incident which would have led to countless civilian (and likely slave) deaths at the Bilbringi ship yards by detonating a baradium fission device, in an attempted to cripple the Imperial fleet production. The pilot never went through with it not out of some moral sense, but because he fell in love with the ship he was going to use as a remote bomb. There were other such incidents throughout their insurgency.
There are other extreme acts taken on the Rebel Alliance's behalf, again mostly directed at military targets but having civilian casualties as consequences. The raid on the Imperial Research facility on Danuta would have been a bloody battle in the surrounding city if Kyle Katarn hadn't defected and joined the Rebellion when he did.
However the goal was never "support the Empire in voice or deed and die". Which if the Rebels had been a Terror group, would have been the primary goal. In other words "kill anyone and everyone who disagrees with your point of view."
