Now you could argue (and this is not impossible to defend) that humans aren't evil, if you use a particular definition of evil, something like "actively, willfully working towards increasing aggregate pain and misery".
You could argue that even for most of history's greatest monsters, misery and death were side effects, not a necessity - Genghis just wanted to rule, he only burned down cities that resisted; Hitler wanted the German people to prosper, not caring about the detriment it would have on other nations, etc. Every real villain thinks they're in the right, thinks their cause is just enough to justify atrocities. Generally you don't see people doing things "for evil's sake" like with kids show villains. Everyone thinks history will justify him.
But anon, what of those without grandiose designs, like serial killers and the like? You could argue they're mentally ill (and they generally are) and therefore are 'faulty products', not representative of baseline humanity.
So you could argue that most real world monsters either believed they were ultimately doing good/destroying evil/their cause was important enough to justify monstrosities, or that they're deranged and therefore can they truly be considered evil if they have a malfunctioning inner moral compass?
But that's a LOT of hedging and nuance, and even then open to interpretation.
Saying "humans can't be evil, only monsters are evil" either implies actual monsters are real (in which case go see a doctor) or, ironically, dehumanizes actual humans - "oh you did something I consider wrong? You're no longer human"... which is the exact argument genocidal regimes used (Jews/Communists/[insert group here] is not really human so we shouldn't treat them as such). Dangerous precedent, anon.