Yes,
For as he thinks within himself, so he is. He says to you, "Eat and drink!" But his heart is not with you. - Proverbs 23:7
but I say to you that everyone who looks at a woman with lust for her has already committed adultery with her in his heart. - Matthew 5:28
So thinking about it, has the power to influence you to do it just as committing adultery in the heart. If someone thought of killing someone for pleasure or hate or sadness and was contemplating about it, wouldnt that be sin? So yes you should try to think positive my friend.