Put me firmly in the camp that thinks it is better for lightsaber colour to be, at core, an aesthetic rather than a product of being a "good guy" or a "bad guy" . Like, we know that how Jedi traditionally found their khyber crystals is kind of Force-magicky. But even then it should have come with a side of "you can intuit what colour a khyber crystal will produce before you use it" leading to people avoiding red crystals rather than red crystals themselves being "evil" somehow.
Anyway - I thought Finn's generation of Stormtroopers were supposed to have been given a crash course in anti-lightsaber combat. They were, by definition, expendable so it would not have been surprising that their safety when using an extremely self-dangerous weapon was not high on the priority list. Presumably some foolish or inattentive trainees would have bisected themselves, but Finn one of the properly careful ones.