Jinx is not primarily interested in killing people.
She is not going out of her way to brutally splatter the guts of an innocent guy all over a wall. But if she sees a factory for fireworks at midnight, she definitely will blow that thing up.
Her main goal is fun by blowing up stuff, that people get hurt is collateral damage, but she doesn't do it on purpose. And that aspect of her personality is still kept in her Star Guardian theme. She still just wants to have a blast by blowing everything up. The difference is just that she added sparkles to the explosions she caused.
Meanwhile Evelynn: she is living from the pain and despair of people, giving them false promises just to rip their flesh apart. She is way darker than Jinx will ever be and I don't think that someone whos primary personal trait is being a sadistic torturer really fits into a Star Guardian team.
An anti-Star Guardian team would actually be interesting. Give the Star Guardians some intelligent enemies to fight instead of just the mindless void monsters. Maybe it is even an opportunity to implement Urgot into the Star Guardian universe without making him an actual Star Guardian. I think this has potential.
But as long as I am breathing, I will always protest against Evelynn becoming an actual Star Guardian, the same goes for Urgot.