Oh boy, here we go again... Riot makes an 'evil' character, and everyone feels that it's their duty to point out that she's actually a soft muffin inside and just misunderstood, branded as a heretic unjustfully.
And yet her lore states that since her early years, all she did was question and deny the worship of the sun, searching for evidence to disprove it. By adding two and two, we could clearly see why would she get on Solari's bad side, storming into their temple while wielding the relics of their ancient counterparts. But when they told her she was wrong, she didn't wait for her execution, and killed everyone instead. And not only that, as the lore says, she still is doing it.
^ That entire passage, Diana stirred up things by herself, four times in a row, without the Solari doing anything to her in the first place.
Then, not to mention, Solari's relics are armor, shield and a one-handed blade, a set very befitting to a protector; Lunari relics are light plate armor, and a two-handed sword with blades attached everywhere they could fit on the hilt, clearly a much more offensive solution. Same goes for their powers, as Leona's are more protective and beneficial to allies, while Diana's are all about killing people fast and furious, et cetera.
But then, people say - "But Diana sought everyone's approval, and they violently rejected her! Clearly she's a martyr and was traumatized by the evil Solari!" And here they are wrong again. Clearly, with the massacre she did at the temple, our lunar child is anything but a martyr. Indeed, she sought approval, but the ways she sought it through were beyond extraordinary - with her constant disbelief of the Sun, she would fit just as much as an atheist within a family of mormons. They didn't hate her, and accepted that the girl had her flaws, but Diana didn't want tolerance, she wanted for people to actually change their beliefs (that they coincidentially carried through centuries on their secluded Mt. Targon peak). And when they didn't, she decided to convert them even if it killed them, literally.
Religions are serious business, especially at the timeline point where Runeterra is. Trying to delve deeper into a character and find new edges to their personality is wonderful, and people should always do more of that; however, an unprovoked massacre can barely be justified, if at all. Diana is one of those cases when looking for good things will fall on deaf ears... and a huge crescent blade.