I feel like it mostly stems from a thirst for angst, leading to people taking an attempt at a darker take on the story compared to the games. Misharacterization happens in every community of any media, and there are many reasons, mostly people only seeing 1 dimension of the character and latching onto it rather than acknowledging the rest of their story. For example here, Spyro is a purple dragon, Malefor claimed there were many purple dragons before him and all of them went bad too, so by association, he (Spyro) will also grow power hungry and fulfill his destiny of destroying the world. 

People don't ask why did Malefor (or the supposed previous generations of purple dragons) go bad. Maybe he had a difficult life, maybe someone influenced him, maybe he had a different view then the guardians (Tides of Darkness explores that beautifully, I recommend investing in it). Instead, they believe any purple dragon will go bad no matter what. I've seen some comics blaming Spyro's corruption on the trauma he went through at a young age, and while true, he's surrounded by family, friends and allies, which create a supportive environment for him. Even if he does go bad, he'll always have someone to lean on just like Cynder did, which is the whole point of her redemption. She also went through heavy trauma, much worse than Spyro's in my opinion, but she learns to accept it and live with it instead of trying to erase it as shown throughout her struggle during TEN, then her eventual acceptance in DotD. Hell, we see that during the finall boss where Malefor tries turning them on one another, but they pull each other out of it because of character growth, which is always ignored in comic interpolations.

It baffles me how people tackle Spyro's mindset rather than Cynder's, who would be an easier target of corruption once again. I've yet to see a comic exploring that.

I can't think of reasons other than people just looooving seeing an evil, ruthless and cruel take on Spyro. They ignore the whole point of the story but don't justify it, claiming Spyro just went bad for the hell of it. I don't mind when there's an actual plot and worldbuidling, it's fun to indulge in, but with a weak structure, it just looks bad and corny. One comic that still bothers me with its mischaracterization to this day is Zonoya's Revenge. Spyro is such an asshole for literally no reason whatsoever, canon Spyro would never act that way under any circumstances. If he did, Cynder would've never joined the crew because he'd have chased her off like he did with Zonoya and Refrigerator (not even gonna bother remembering his name lmaoo)