Well to start with maybe broaden your idea of evil. Just killing and murdering is evil but there are lots of other ways to be evil.

What about slowly turning good characters bad by slowly encouraging them to do small wrongs. Like convincing them to pick someone's pocket. Or justifying theft and other petty crimes.

What about looking the other way when criminals commit crimes and just saying... Not my problem.

Start thinking about long term goals and then add narcissism and selfishness to every choice.

Villains don't always think they're evil and don't often do evil things just because they're evil.

Often they think they're doing what's necessary for what they consider the greater good. For example, an evil character might think that strength is key to survival, is a higher good, so the weak should be punished or eliminated. For example, if you come across a camp of nice but weak innocents, you might side with those attacking them, because you don't believe the weak deserve to survive.-

Or maybe they think they're being kind. Those poor villagers were going to get killed by the monsters anyway! Let's be kind and make it quick by helping the monsters make a quick job of it. Let's kill the eldery crone or the children who are hungry. It's a kindness.

Or they might follow a god and do things to appease that god. Serving your god is good, so anything you do that pleases your god is good, whether it's human sacrifice or killing fluffy animals. Similar to the weak villain, who is too scared of their god or master to oppose them. Like Renfield in Dracula, they (think) they have no choice but to do the evil things their master wants them to do. Often they're in denial about how they actually enjoy doing these evil things.

Then there's villains who have no morals or who are nihilists. They do evil things because they're bored, because it's easy, and/or because they don't believe it matters either way. Burn the village down? Why not? Everyone dies anyway. Corrupt someone good? That's a fun game to play. Someone powerful but good comes along? I'll help them, because to do otherwise would be dangerous or disadvantageous.

Then there's villains with delusions of grandeur. They think they're better than everyone else and will do everything in their power to ensure that anyone who might be better than them, gets cut down to size. Or they like to get involved in evil plots and schemes, because it means they get to feel like they're influential. Someone wants to overthrow a kingdom? Time to get involved so that people know me as having changed the course of history.

Or there's the evil characters, who are deeply cynical. The world is evil, nothing is fair, I'm just doing what's necessary to survive. If I don't kill/rob them, they'd do it to me. They probably deserved it anyway.

I get it. I was never really good at evil playthroughs, but one thing that really helped me get a good evil playthrough in bg3 was inventing a motivation for my character to be evil instead of just always picking the evil option for the sake of it. I just thought of a little back story and personality traits that would make picking the evil options more logical. There are a lot of motivators to do evil things other than just to cause as much harm as possible. Also its good to keep in mind, that just because your character is evil, doesnt mean they always have to pick the most evil option.

I really struggle with evil play throughs in most games. Rogue Trader is a notable exception for me, as I'm doing the whole "save humanity at the cost of my own humanity" and that's the one approach to evil that I've actually been able to play and enjoy in a game

Meanwhile I was streaming BG3 earlier and killed a child way too casually.

Hopefully you're having fun at all the chaos of it all though. Because I'd be laughing a fair amount just out of "How does this keep happening?" Sounds like you've got an awesome and memorable game going!