I have a joke when drinking wine. The taste, texture, aroma and experience are all good, but someone should fix a flaw in it. Because all wine seems to have this problem where it stops working. There you are, one moment listening to the greatest hits of the 1980s and thinking how great everyone around you is, and the next you are sitting, feeling grouchy and disconsolate with a headache. This is a problem with other substances of use and abuse as well. You can not permanently stay in a state of ecstasy and exhalation. This is also the problem with material possessions and success. Even if it was possible to have all the possessions you desired, the pleasure of it would wear off soon enough.
If you have completed adolescence and do not have a personality disorder, this is something that you have come to accept.
I think, however, our society has one major loophole, big enough for us to drive a truck through, with the truck in question being our narcissism. And that loophole is Love, aka Wuv. Maybe we can accept that we will never be the most beautiful, rich and talented person around us, but the concept of love gives us a way to imagine that we are important and will be able to live in a state of perpetual exhilaration. But unlike making it a goal to stay permanently high on opium or cocaine, the idea that love can provide perpetual excitement is not seen as either selfish or unrealistic, but instead is seen by many as a laudable thing.
This is not just related to romantic love, either. The love of missionaries, whether religious missionaries or secular missionaries is another example. By loving and caring for others, the missionary is a hero who is connected to some higher power, whether it is God or "social progress". In this case, the narcissism is just connected to something larger, and more hidden.
This being explained, we can return to the title of this writeup: entertain evil thoughts. Although I don't know if "evil" is the term I would use, since "evil" is too associated with something demonic, twisted and occult. These "evil" thoughts are perhaps the most mundane, reasonable things to think about. Life is full of frustration, disappointment, anger, loss, resentment, and admitting what is a low level background white noise of negative emotions caused by everything from a slow-moving line at the DMV to the betrayal of friends is hardly "evil". It makes much more sense to focus on the emotions and thoughts we mostly experience, rather than to try to pretend our lives are about us being either the slayers of evil, or saviors of the world.
Because the truth of the matter is, that emotionally as well as spiritually, we may at some point be dying, afraid and in terrible pain. And to me, while I can easily conjure up memories of being ten miles up a mountain gravel road, dehydrated, and with the realization that my tooth has become inflamed enough that touching my tongue against it sends electrical shocks through me, and I can easily conjure up the feeling that no matter how much I want her to, and no matter how easy it is for her, she is not going to answer her telephone. But it is much more elusive for me to remember thoughts of being understood, in harmony and at good terms with the world around me. A step in spiritual growth is to accept the world as it is, not as we wish it to be, and it therefore means that we should accept the world, and ourselves, for the evil within.