I think the big sin behind the misunderstandings about spiritual warfare is the lack of humility.
If one is not humble, if one is not pursuing holiness hourly in some way, even with small prayers, one will fall into sins of curiosity and pride, thinking one can deal with things which may not even be our business.
Because of the prevalence of the occult, more and more people are being possessed. One can listen to Fr. Ripperger on this.
As we are all sinners, as we have defects and imperfections, satan will use anything against us, including past sins.
Humility is the needed virtue for all growth and for all spiritual warfare.
If you only have time for one talk, I recommend this one. And, if you are being stubborn and still do not see the evil in Harry Potter, listen to 1.08.36 on or so.
http://www.sensustraditionis.org/webaudio/Tulsa/Newman.mp3