You're missing an important point that is not in Morgana's lore, but Kayle's. Something important that is missing. The city was rioting and was already destroying itself. Morgana apparently did nothing to stop it.
Remember that the lores are going to be effectively from the respective champion's standpoint. From Morgana's standpoint, she had to protect. Was that really what happened? Probably not. She probably could've stopped him in a non-violent way or protected via a shield. Instead, she incinerated him and in the aftermath, the whole city went into chaos and rioting. Kayle came to restore order and, from her perspective, saw her sister as the instigator. If you had a crazy power filled sibling that appeared to have killed someone for no reason and was causing an entire city to revolt and act on their dark emotions and instincts, wouldn't you try to stop her? For the good of the world and to keep the destruction from spreading?
There's an important thing that a lot of people are ignoring from Kayle's lore -- Kayle was compassionate too. She didn't have to listen to Morgana; she could've smited everyone that violated the law despite what Morg pleaded. She didn't though and it was her followers that didn't like this. She chose to be merciful to those that pleaded to her sister. If that isn't showing that she isn't some villainous harbinger of destruction to anyone who is out of line, I don't know what is.
Honestly, it feels like the whole Boards wants to rant about Kayle being evil and Morg being good without looking at the full picture. I had the same arguments with a bunch of people that believed Ezreal was a psychopath/sociopath and only once they saw the full picture did it become clear that he wasn't.
Kayle and Morgana are a split of an aspect and it makes sense that they are two sides of one whole. The Star Wars analogy is good with the whole Dark/Light side. The Aspect of Justice is two parts -- lawful and compassionate, strict and merciful -- at the same time. Being twins born of an aspect, each inherited one of the traits. Kayle is the lawful, strict, righteous justicator that has to hold the higher standard at the cost of her own emotions, feelings, and desires. Morgana is the merciful, human aspect that recognizes the burden that humanity bears and does so at the cost of realizing what is actually right and necessary to keep the world in order and justice preserved. Together, they balance each other out, but without each other, each falls prey to the extreme of their side. Because we are humans, we readers will automatically sympathize with the more human aspect and thus assume they are good when that is not necessarily the case.
Neither are good, neither are bad. Both are just doing what instincts they were born with and being separated by the treachery that Ronas committed, they can no longer balance each other as they always did.
Kayle is "good"; she uphold the laws and justice of the universe and if she doesn't the world will descend into rioting and chaos resulting in nothing but death and destruction. Kayle is "evil" in that she smites people without question to cleanse the sin that she, being righteous, cannot tolerate and levels whole cities to bring order.
Morgana is "good" as she understands humanity and has a deep connection with human emotions that allows her to understand and help those that wish to atone. Morgana is "evil" in that she does not stop the chaos and focuses too much on her own emotions, bitterness, thoughts, and feelings to see what things need to be done in order to keep the world from anarchy.
Does that make sense?
Bruh. . .
<---- that guy is evil