Idk which I dislike more, the people who apply such a double standard or those who don't. Because there are in fact a sizeable contingent of fans who do say the Rebels/Resistance are morally grey because they kill Stormtroopers and also destroyed the Death Star along with all the people on it. Then they jump from there to say Kylo did nothing wrong uwu, or at least nothing more wrong than his parents, Luke, and the other heroes. Because torturing prisoners and wiping out entire populations is totally OK in war and morally and legally the same as killing enemy combatants hunny the Geneva Conventions say so*

Galaxy-brain takes aside Finn arguably has more reason, not less, than most not to hesitate when he fights his former fellows, as you point out. This is in the extended canon but that poor dead Stormtrooper in the opening scene who marked Finn's helmet with the iconic bloody handprint? In the junior novelization directly leading into the movie that guy opened fire on starving, desperate miners who were on strike. He was the least capable member of Finn's fire-team, but it doesn't take much skill to mow down unarmed people with a gun. Finn knew exactly what kind of people he faced when he was in battle against Stormtroopers.

* They don't