I personally focus more on numerology (and Astrology as that's what I was trained in before I picked up Tarot) when reading the cards as I don't like to get too caught up in any particular lore; I find that things can very quickly become convoluted when one places too much emphasis on one (or many/all) philosophical/spiritual stories and the message can get diluted rather than become clearer.
I agree with what others have said here - that card 15 is about ambition, wealth, and physical satiation. I believe most modern Tarot guides don't typically associate this card with good/evil but rather talk about these topics as strengths as well as vices (I have seen card 15 as promising some level of success in career endeavors for example as Astrologically it is associated with the driven energy of Capricorn). But as others have already mentioned, ultimately, it's energy carries a warning/a test (much like Capricorn's planetary ruler, Saturn. Scorpio/Pluto is associated understandably with card 13 Death - NOT the Devil as some might think) and I don't see how that can be misunderstood.
In some traditions, the Devil is less about helpless addiction to the material world and more about deliberately embracing the physical. It means accepting the transience of life and as the saying goes, 'sucking the marrow from the bone, not choking on it'.
I believe herein lies the Saturnian test: 1+5=6 the number representing the
process of achieving harmony. 1 = individualism, self-centered interest (because of the lack of recognizing the "other" as part of the self), ambition and exerting one's willpower. 5 = an upset of that which was once stable/balanced causing discord or upheaval, chaos, and ultimately the opportunity for expansion.
I don't believe this energy is saying that it's a good idea to deliberately embrace the physical or that it is ever in the querent's best interest to fully succumb to the temptation of fully living in the physical. For me, it is important to stress the test aspect here. Part of the maturation process (again here we find Saturnian energy) is to recognize that we do not live in a vacuum and that our actions, desires, and willpower affect the "other" (and ultimately ourselves) for better or worse. It is a slippery slope and is a difficult concept for us as humans, being grounded in the physical, to grasp but it is ultimately an important one for growth.
EDIT: I also wanted to mention that with Capricorn/Saturn and the number 5, there is a very serious energy/gravitas associated with this card which doesn't fit with either a "bad boy" or "misunderstood rebel" characterization. It is a severe and important stage in which the querent's actions can have a serious impact on him/herself, others, and ultimately his/her journey toward happiness and fulfillment.
That being said, my big disclaimer is that I am a staunch proponent of reading intuitively, so any gut feelings you have with this card over time should ultimately be how you come to understand it's energy - books, lore, and other Tarot scholars can only provide guidance to help you conceptualize how the energy of 15 is being manifested but they certainly aren't the end-all interpretation. Many experiences with this card have come to shape the way I view it and the way I interpret it when it appears in a reading, so perhaps spending more time with this card in your own practice will help you answer any questions you have regarding its symbolism in your readings.
Also, I highly recommend checking out the Tarot of the Name by the Tarot School. It is a dramatically different interpretation of the Tarot system that definitely helped me to strip away too much superfluous information and provided a unique perspective devoid of traditional doctrines.