Part 1: What evil is:
I'm not just talking about a "mere" clinical psychopath or sociopath who doesn't feel guilt and isn't restrained by social norms. First of all, people who don't feel the emotions can decide to be good anyway, or at least not pointlessly cruel for no benefit. Second, everyone partly fulfils the definition of psychopath (see this TED talk for details). Saying that psychopaths are the problem just doesn't work.
I'm also not talking about people who don't know any better, like medieval soldiers who are taught honorable fealty. I'm not even talking about people who theoretically could do otherwise but Moloch would ruin them if they tried. I don't mean victims of circumstance.
What I mean are the many people who know that some things are damaging to someone else, gain no tangible value from doing them (or even expect that their life would be worse off!), know it is not a virtuous act, and do the harmful acts anyway without expecting future good to come from it.
People commonly have a terminal value of dragging other people down. Consider a game theory study by J. Sayer Minas, Alvin Scodel, David Marlowe, and Harve Rawson. The study itself is behind a paywall, but it's described in the book Prisoner's Dilemma by William Poundstone. People would often reduce their own prize if it means that their opponent's is reduced more.
Don't think this study is big enough to be representative? Neither do I. That's fine, though, because there are many, many, MANY more pieces of evidence from (almost) every internet troll, bully, and rapist, and many other criminals too. I'm not including on this list domestic abusers, since they often get rewarded with things like the spouse's money, or dinner being consistently made what and when they want. Remember, I'm not just claiming that some people are evil when it's convenient for them, or intrinsically apathetic. Many people would, when making a choice between harming a stranger or doing nothing, would harm the stranger. My point is that people often are innately evil, not a majority but not a negligible percentage, and saying that everyone is good inside is very, very wrong.
There is some good news. People really can change their behavior and values. Someone being evil now doesn't mean they always will be. But, if you aren't convinced yet, let me tell you specifically the harm done from thinking that people are good inside.
Part 2: Harm done by evil, and more harm done by denying it
Bullies go far beyond insults or "stop hitting yourself" stereotypes. I won't describe the specifics of what I went through (and you wouldn't want me to), but I'll tell you the results. I was so stressed as a child that my hair started to fall out. Until a few years ago, I've been yelling in my sleep almost every night. I have never been in a war, but PTSD perfectly described my symptoms. When I read the webnovel Worm, I considered the bullying described in the backstory to be completely realistic*. The situation was worsening to the point where I started seriously thinking I might need to do an Ender Wiggin-style killing.
I'm not alone either; In stop confounding yourself, Scott Alexander describes a bullying study which found that "In fact, the frequently bullied kids had nearly twice as much psychiatric disease, were twice as likely to attempt suicide, were twice as likely to drop out of high school, and even had double the unemployment rate. Worse physical health, worse cognitive function, less likely to get married, et cetera, et cetera." When you consider that a bully "crops up untaught and unbidden in near-identical form in schoolyards from Los Angeles to London to Lanzhou", it becomes apparent that this is a massive problem. (Sorry Scott Alexander, you totally fell down as a rationalist when you saw one of the biggest effect sizes ever, in a study that controlled for so many things, and still thought "hmm, i doubt being tortured regularly for a decade has a long term bad effect.")
Fortunately, psychologists are aware of the problem and are on the job! They have done a few studies, and found:
Wait what?
What planet are these psychologists from where if you walk away from a bully, they suddenly become stuck in place and give up? For that matter, has any bully ever been stopped by talking to them? "Bullying is wrong." "You're right, I won't torture you".
Actually the effects of listening to advice was worse than in this comic strip. The authority figures in my life had read up on the "studies", and determined that, in fact, the best way to fix this problem was encouraging interaction, talking it out, and reconciliation. Authority didn't just fail to solve the problem, they made it worse by encouraging interaction.
Disclaimer: this section doesn't advise a return to corporal punishment in schools. Almost all corporal punishment would be because of sadistic teachers, enforcing rules that shouldn't have been made, doing something inconvenient for the teacher like contradicting them or "talking back", or, if somehow they are able to successfully punish bullying, hit the victim just as much as the instigator AKA zero tolerance. Ending corporal punishment was a big success because it replaced directly harmful actions with actions that don't do much.
Part 3: Evil is common
How would someone go about finding the rate of evil? Crime rates don't work because evil people often use legal methods to cause suffering, and many crimes have complex motives. And it's not like you can just run a poll asking people if they if they maliciously harmed a stranger for no gain, right?
Actually you can. This poll asked people if they did "malicious online activity directed at somebody they didn't know", and 28% said that they did. Let's subtract the lizardman constant, and then subtract a few more percent because this might include some people who didn't really mean it and would have stopped to think if they weren't mind-killed by a political or religious topic under discussion. That leaves 15-20% of adults reporting malicious behavior toward a stranger. 15% being evil or having been evil in the past seems like a reasonable estimate based on my life experience.
What about bullies? I don't know, since the poll results are all over the place. I tried looking through the results, and, well, this summary I found about sums it up:
If I had to guess why, I'd say that for kids and teenagers, the lizardman constant for polls goes up far beyond 5%. The comic strip "Zits" once ran a strip where the teenage main character gave the most ridiculous possible answers to every question on a poll while giggling, and then the news reported "2/3 of high schoolers believe Europe is a planet". I've met teenagers and this doesn't seem unreasonable. However, Scott Alexander said that a bully "crops up untaught and unbidden in near-identical form in schoolyards from Los Angeles to London to Lanzhou", one of the few things that he got right in that article. Based on the prevalence of bullies in fiction set in schools, this seems reasonable.
I know that people have a lot of corrupted instincts that cause them to treat enemies badly, but bullies and trolls target people who aren't enemies. Also, these are accurate behavioral tests because there is no significant enforcement or reward, so the behavior would be usually intrinsic. Therefore the rate here would probably be accurate for determining the rate of evil.
Part 4: A few more notes:
People can be evil in general but not be cruel to their friends. Don't think evil=always 100% as cruel as possible in every circumstance.
Methods to control or prevent bad behavior would need to be a separate post. However, the pattern isn't just "treat them well and they will reliably grow up well".
Sometimes people who were evil stop on their own, but often they simply continue. You can't reliably "wait it out".
*By the way, here's the bullying mentioned in Worm. (It's not a spoiler because it's mentioned in the first few pages, and is what starts the main plot.) The main character was regularly bullied (conventional methods) by some girls, but then the bullying settled down and the lead bully apologized and became friends. Then, once she had the main character's trust, she betrayed the main character's secrets, and then the girls stuffed her into a locker filled to the brim with used tampons and locked her in overnight with that biohazard. (Wow, no wonder it's one of the most popular webnovels). No bullying I or anyone else I know has experienced was that bad, but the point is, bullies can go far beyond name-calling or even hitting.