Writing Evil Characters (that are *human*)
1. Give them a motivation
If the character is just going around killing people for fun (although there are sadistics who are like this in real life), it's hard to make them memorable, especially if they're one of the main characters. Give them motivations; why are they doing what they're doing? What made them this way? If something previous or present in their life changed, would they still act the way they do? It's helpful to include a tragic backstory.
2. Make them human!
The character might do things that aren't considered human, however you still need to give them human characteristics. What positive things are there about them, if any? (They're good at cooking, have great attention to detail, etc.) Do they like what they're doing (or how their personality is)? Would they want to change themselves? Mostly, give them vulnerabilities. Everyone has vulnerabilities even though you might not know yours yet. The character might be terrified of anyone with their hair in braids because of their childhood, or you could give them a simple fear like they're scared of spiders. It's more effective to have a reasoning behind their vulnerability and fear. That way, the reader understands their actions better and can better connect to them as a person.
3. Don't make them easily defeated.
Most writers want the main character to be able to defeat the evil character. But don't make it predictable or obvious they're going to win; maybe they land the MC in the hospital, badly injured, or maybe the main character thinks they defeated them but suddenly their backup appears. Also, when the evil character makes a mistake that benefits the MC, make it believable. I can't think of any examples for this but I hope you get the idea.
Overall, if the evil character gets defeated, you want the reader to think, "He deserved it, but I wish he would've been able to relive a good life where he's not driven by his childhood."
