Why religious people refuse to believe that evil exists?

God/god? Which God/god?

Some people think that God will punish them for believing in evil spirits, so, there you go.

Though effectively atheist (prefer the term areligious - I don't agree enough with any religion to think it's a good idea, but don't discount the possibility something we'd call a god exists) I don't believe in good or evil either. Think there's only actions and consequences.

If a soldier goes to war and kills people is he evil like any murderer? Is a non-soldier who picks up a weapon and kills a soldier 'good' like the soldier (if not evil?) Or is killing killing regardless of the context? Both Einstein and the Red Baron both called killing in war 'murder.'

Is a trophy hunter (using a recent example) who kills just for the fun of killing 'evil?' Is a hunter who kills to eat, or make the meat available to others as meat 'evil?' Or do both simply kill and neither's good or evil? What about the wild animal who kills to eat or for fun (bottlenose dolphins have been documented as killing for 'sport' or 'fun' smaller porpoises, but not eating them.) Are these dolphins 'evil?'

Evil is not a thing as much as cold or dark are not a thing. They are the words we give to a relative lack of a thing. Evil is not the opposite of good, but rather the name we give to a lack thereof.

What we may call evil, other cultures may call good, relativity is a wonderful thing.

Now you are getting into something too deep and off topic here. The context is that if someone believes God, an ultimate "good", then, the opposite of that must also exist; otherwise, there's no contrast.

You are speaking of ethical relativism which is going to be an endless debate. Regardless, I am sure you believe to some extent that there should be some fundamental difference between good and evil that's not depending on the perceiver. Good, or compassion is instilled in sentient beings. (Animals, by the way, are not sentient. At least there's currently no scientific evidence that any animal are sentient.) If all is relative, then what's stopping strong nations from destroying the weak ones? Then Obamacare would never have passed because who gives a f$#% about those who can't afford healthcare.

You speak well my friend, but why must there be contrast?

The problem you raise with implanting the necessity of contrast is difficult to get over. how can a god of ultimate good, create a being of ultimate evil, and still be considered ultimately good?

There must be a contrast because without such, every choice is indifferent, because there will not be a good or bad choice. There will also be no difference between living and death if they are not the opposite of each other. Many modern philosopher had envisioned a place where only happiness exists (a Utopian society), but has ultimately failed because happiness is undefined without its opposite.

For the sake of arrangement, if, say, an ultimately "good" god creates lucifer. At the time of creating him, lucifer was good and did a number of good deeds and was god's favorite. However, many eons later, lucifer turned into the greatest evil in the universe. Can we say that god is evil? I don't think so, because at the time of creation, lucifer was good. It's much like the mothers who gave birth to Hitler or Stalin -- are they inherently evil?

Also, If god is omniscient, he knows that lucifer is going to turn. However, if creating lucifer is a necessary condition to achieve a greater good, is that still evil? We as mortal beings cannot see a billion years into the future, so obviously we cannot envision what that future will be like.