IMO she's always been evil, or as others would like to put it - "insane." In fact I'd go so far as to say that she's one of the most irredeemably evil champions in the entire lore, bar-none.
Why do I believe this?
Because it seems like Riot purposely wrote her with very little - if any - positive qualities that remotely justifies or validates any of her behavior.
League might have a plethora of "evil," "villainous," "morally questionable" and/or "mentally insane/unstable" champions in its roster, but even then, many of these champions were also intentionally written with at least some degree of positive, relatable, redeemable, sympathetic, tragic, understanding, humane or logical reasons that explains why they are the way that they are, and in-turn, makes them a little bit "like-able" for their playerbase.
Like, maybe the "evil" or "villainous" champion had a horrific, traumatic childhood, and that explains why they act they way they do. Maybe it wasn't even their entire childhood, but a single, traumatic event that triggered their slow spiral into insanity, like the loss of their parents, a child, a close friend or their lover. Maybe they were back-stabbed or betrayed by a loved one, or by the people, order or nation they once served. Maybe they were forced to take on the role of some evil totalitarian leader in an effort to preserve life and order for the greater good. Or maybe they're an evil leader who genuinely believes that they're doing the "right thing" for greater good of the people in their own warped and twisted minds. Maybe they were unjustly exiled or outcast from their society, and decided to become evil anarchists as a form of backlash. Or maybe they were driven insane through grueling, inhumane torture or isolation. Maybe their minds and hearts were permanently corrupted by an omnipresent, evil force - like the World Runes, or from Void corruption. Or they could be addicts whose sanity and general humanity deteriorated overtime in an effort to further validate their addictions. Or maybe they were once normal people who fell victim to the corruption that often comes with extreme wealth or power.
Either way, the point is that the vast majority of League's "evil" or morally-grey" champions have valid, believable reasons & explanations for being the way that they are. Even the demons and the voidborn - some of League's most seemingly "evil" champions - have to corrupt, kill or eat people out of necessity. Demons can only sustain themselves on the souls and emotional suffering of their victims, while all Voidborn feel innately compelled to satiate some kind of insatiable drive or "hunger." If either of these two groups somehow managed to drop their corruption/killing/eating/etc act, then they'd surely die. Therefore their "evil" actions aren't really all that different from the violent actions of a tiger or a shark, or any other apex predator for that matter. They're just doing what they need to do in order to survive.
But Zoe doesn't really fit the bill for any of these tropes. She didn't have a horrific childhood. She didn't experience some traumatic event that permanently affected her for the rest of her life. IIRC, There's no indication in her bio or her in-game quotes that she misses her friends or her parents or anyone from her early life as a mortal in her bio, short story or in-game quotes. She didn't seem to experience any real peer pressure or social ostracization, either. Sure, the aspects might order her around to do some questionable things - but at the end of the day, they aren't threatening her with her life by any means. They even give her enough leeway to make her own choices on such matters, which can leave a profound impact on Runeterra's people. And those choices - her free will - are only driven by personal amusement and self-interest... and Zoe seems to get a lot of amusement out of watching people suffer.
IMO, she's one of the few champions in the lore who truly fits the definition and category of pure evil. That is to say, born-and-functioning primary psychopaths who willingly and carelessly hurt, torture, manipulate, kill and falsely "befriend" others entirely out self-interest personal amusement with zero moral repercussions whatsoever. I can only think of two other champions who'd fit such a vile label - and those champions are Singed and Mordekaiser.
So yes, she's always been "insane" because she's a fucking psychopath.
because a guy she likes
has a crush on that girl.
