There are rare cases of evil acts being the work of demons, but for the most part, as the other posters said, it is our own evil desires, that bring about sin. It is sort of a birthday gift from Adam.
The OP was about thoughts, not acts.
Also, if the whole unchristian world is, for the most part, being led by their own evil desires, and not Satan, to do their evil work, then how come 1 John 5:19 tells us that the whole world is under the control of Satan?
Ephesians 2 also says,
vv. 1-3, ESV
And you were dead in the trespasses and sins in which you once walked, following the course of this world, following the prince of the power of the air, the spirit that is now at work in the sons of disobedience— among whom we all once lived in the passions of our flesh, carrying out the desires of the body and the mind, and were by nature children of wrath, like the rest of mankind.
that the "passions of our flesh", "the desires of the body and the mind", are in control of Satan in the non-christian: Thus attributing the responsibility of evil in the evil nature of men to Satan.