Suppose a trolley is barelling down a track towards five tied-down people. If it continues, it'll kill them. But, you have access to a lever which would send the trolley down a side-track instead. But the side-track also has one person tied to it. By pulling the lever, you are effectively murdering that helpless, innocent person -- but you would save the other five. If you don't touch the lever, then you are just an innocent bystander who happened to see a quintuple homicide.
In this case, if you insist that murder is evil, and you refuse to be evil, then you have no choice but to let the five people die. But if you allow that murder is sometimes okay -- which is a utilitarian perspective, in the sense that you care more about the ends than the means -- then it becomes much fuzzier whether murder is inherently evil. Indeed, as soon as you give an explicit definition for evil, it becomes very difficult to defend it. The fact of the matter is, the good/evil dichotomy is false and artificial.
Especially if those concepts are touted as being all-encompassing, holding without exception or being superior to other concepts.
Is that evil? Or is that justified?
If you delibrately disturb harmony, or deny someone their rights, or take away something by force that isn't yours, you need to be stopped. Because one thing would lead to another and life as everyone knows it would become a living hell.
Step back and look at oppressive regimes over the course of history. They all have traces of evil in them. It's that simple. I admit that good for one might be evil for another, but there's a basic set of things, I guess, the majority can agree on about what's good and what's not.