You might start out by telling him that everybody on earth has bad thoughts, sometimes VERY bad thoughts, and that it's normal and he isn't a bad boy and just because he thinks them does't mean he'll act on them. Express unconditional support and love no matter what he thinks or does. Remind him, depending on your religious views, that he can pray about it and ask for forgiveness when he has bad thoughts.
That said, any significant change in behavior should be taken seriously, especially if it lasts more than 2 weeks. I do agree that he may need some counseling. It appears that he may have been holding in some of these very stressful feelings for a long time until something triggered the need to "let them out." Living with those negative thoughts for any length of time before "confessing" must have been very stressful and worrisome for him. He could certainly benefit from an objective outsider to talk through and help him learn to cope with these thoughts, find out why he is having them, and what changes he can make to lead to more positive thinking.
have you ever heard of people being possesed by an evil spirit. Not to scare you but it does happen. I know because it happend to someone I know. She was doing witchcraft, tarot cards, and spells. She all of a sudden changed and was always scared something was going to happen to her and voices were telling her what to do. She was seen by a priest and he did an exercisim and she really did some freeky stuff while the exercisim and got a little better but she is still not normal and I believe in what it says in the bible that sometimes people are possesed by several spirits and that they probably didn't get all the spirits out just some. But anyway, I suggest that you take this serious and believe your child is really trying to tell you something. I also believe in all this stuff because I've seen to much of the unseen world to not believe. I'm a strong believer in God and I don't know if you are but God is the only way for him to get better if he is possesed. I would seek someone from a church who is confident in dealing with this stuff.
Maybe the preaching tape freaked him out and he thinks hes going to go to hell if he doesn't confess his "sins". What did the tape say that made him cry?
He may be just very anxious and beginning to develop an anxiety disorder over it. You should take him to a psychologist as soon as possible. I'm sure he will be fine as long as he gets someone to talk this through with..