In ACIM the idea of evil is defined as 'ego', more or less. Not just individual egos but also a collective one.
The ego uses temptation to distract you from the truth. It is capable of creating scenarios in which it seems like you want and enjoy things which you would normally not want or enjoy.
It is also very cunning in that it can use your very own weaknesses against you. If you think that some person or project or activity might be 'the answer' and that it will save you, even though it's not true, the ego will encourage you to pursue it. And it knows you are likely going to go along with it because you yourself are the one who is believing it will work, or is what you want.
I have only recently discovered that my own ego has been 'playing me', using my own self deceptions to lure me away from the truth. It's particularly bad if I am very mistaken about something, like I think I'm onto some kind of solution or really great thing, but really it's another ego bullshit wild goose chase which will have me 'seek and do not find' for a long time.
In other words, if I choose to believe in an illusion and I give it faith and am convinced of its supposed value, this becomes a blind spot in my awareness. My ego comes along and says "why don't you do that thing, you know you want to, you know you like it, you know how appealing it is". And then I'll be all gooey eyed with hypnotic spirals spinning in my eyeballs while I salivate over this very appealing, attractive opportunity. All the while, my mind is completely distracted from God.
Notice though that the ego thus really has no power of its own. It can only use the things I have agreed to as a way to tempt me away from the truth. If it weren't for me getting gooey eyed over some new piece of technology or some amazing movie or some pathway that seemed really enticing, the ego would have no way to deceive me. I have deceived myself, willingly, and that is all the ego needs.
It can trigger off a memory of these tempting offers just at the right moment, knowing that their allure will particularly hurl my awareness off in a different direction, like some rat in a maze that just caught a glimpse of a big chunk of delicious cheese. Suddenly all my senses are ignited and the unfinished business I have with this 'promising savior' has captivated/captured my attention. I fall for my own traps.
It's good to be aware the ego is actively tempting us in this way, using our own mistakes against us, and trying to get us to reinvest in anything other that what we really are. There is nothing guilty or bad or wrong with enjoying life, but if we are psychologically using it as a false savior, then we will fall for temptation - the temptation to believe the truth is not true and God does not deserve our attention.
Watch out ... for yourself :-) You are the one who gives the ego the fuel it needs, and you thus hire it to serve your false interests. It is merely a reflection of your own self deception. It is not against you but for you. You want it. But you need to want God instead.