But if he only thinks about doing something bad in matters of modesty and holiness, he will feel as though he hasn't done anything, he's not going to have the feeling of how much he has damaged and dirtied himself spiritually therefore the thoughts of sins are worse- their correction is more difficult than the sin- because a person is not aware of how important it is to truly repent even for this and purify himself therefore the stain may remain on him all the time. 