What an interesting thread. What I have learned over time, and I'm not being dogmatic, this is merely my opinion but I do think I am right and my thinking on the subject is based on scripture.
First off, ntrusive evil thoughts can come upon anyone and having this initial thought is not evil or sinful immediately. But it is what we do with that thought when it first pops into our mind that is important.
2 Corinthians 10:3-6
3 For though we walk in the flesh, we do not war after the flesh:
4 (For the weapons of our warfare are not carnal, but mighty through God to the pulling down of strong holds;)
5 Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ;
6 And having in a readiness to revenge all disobedience, when your obedience is fulfilled.../
So we are to take captive these thoughts to Christ. Ok, but what does that mean really? It simply means to not dwell on the thought. Think about Christ instead.
Philippians 4:7-9
7 And the peace of God, which passeth all understanding, shall keep your hearts and minds through Christ Jesus.
8 Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things.
9 Those things, which ye have both learned, and received, and heard, and seen in me, do: and the God of peace shall be with you.../
The peace of God shall keep our hearts and minds. Hmm, our heart.../
Jerimiah 17:9
9 The heart is deceitful above all things, and desperately wicked: who can know it?
10 I the Lord search the heart, I try the reins, even to give every man according to his ways, and according to the fruit of his doings.../
Proverbs 23:7
7 For as he thinketh in his heart, so is he: Eat and drink, saith he to thee; but his heart is not with thee.../
So there you have it. The intrusive evil thoughts can be coming from our own heart. Or they can come from the enemy. They are not random thoughts. But what we think on and dwell on leaves a residue in our mind for awhile. What you think about is what you connect with and who you become. (as a man thinks in his heart so is he). How many times have you woke up thinking about that movie or video you watched last night? Or a song and sometimes it will stick in your mind all day. So thoughts leave a residue. I used to think that, well I'd love to teach that guy a lesson, but of course I wont because I don't want to be that guy or get in trouble, but it's fun to think about and that is ok...That Is Not True. Dwelling on evil thoughts even if not acted out will at some point, go...whoosh Into your heart and becomes who you are. Not good. So how do we get rid of the evilness within our hearts?
I'm glad you asked, scripture tells us how. In Philippians 4, " whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things..../
And if we do think on these things instead of bad thoughts, these thoughts will leave residue in your mind and go into your heart also, and eventually the cup of your heart will runneth over and the evilness within will be forced out and replaced with the knowledge and righteousness of the Kingdom of God and who we will then become.
Now it has already been posted that, it is what comes out of a mans mouth that defiles him. Another scripture is,
James:3:10
10 Out of the same mouth proceedeth blessing and cursing. My brethren, these things ought not so to be.../
Therefore, I believe that man speaks from the heart, and that every single word that a person speaks is either a curse or a blessing. So in laymans language, every single thing you say either serves to build up another or curse another and tear them down. This is very important in relationships. Husbands and wives fight and argue at times. But don't tear her down! And don't even talk behind peoples back about them to yourself or others. Gossiping, whoa. It can curse them. It will affect them and it will affect you also. So if you argue, fight fair. Don't cross lines and speak evil over people! It'll hurt you prolly more than them.
"you so and so, you're useless and lazy and have accomplished nothing!" Not only does it emit a curse unto them but it is programming your heart to be that same bad thing that you spoke. One day you'll wake up to find, wow, things have gone bad for me. How did that happen?
Our words have the power of life and death. They can bring joy or cause misery. Proverbs 18:21 puts it this way: “The tongue has the power of life and death.” The stakes are high. Your words can either speak life, or your words can speak death. Our tongues can build others up, or they can tear them down.
So best to reject those bad thoughts when they pop into your head the first time. Do not entertain those thoughts or that is who you will become and it shall become your reality.
...Or so it seems to me.