Q: What actions or thoughts would prevent one from taking the sacrament? Obviously killing a person, stealing, coveting someone who is not your married other, are all things that would prevent it, especially if one does not have any remorse for their actions but, is there any thing else and what severity are they at? Could just the desire alone without the action cause impurity unworthy of the sacrament?
A: For those who are not members of the church, see this post. For people who are Mormons already, I would answer that taking the sacrament is a very personal experience. I hesitate to give any members of the church opinions on whether they should or should not take it. The only person, besides yourself, who can tell you not take the sacrament would be the bishop. He would recommend that as part of a disciplinary measure for something fairly serious. If you have any sort of question about a specific thing that you have done that you think is serious enough to fall in that category, you need to just ask the bishop about it.
But after reading your question, it seems like you understand about the serious things, and you want to know some opinions about things that are less serious, like desiring to do something bad, or being not right with the Lord in your heart somehow. Luckily, the sacrament prayers themselves come with a pretty good answer. The person giving the prayer describes to God what you will be doing when you take the bread or water. For the bread, it says that those who partake witness before God:
“…that they are willing to take upon them the name of thy Son, and always remember him and keep his commandments which he has given them.”
And for the water:
“that they may witness unto thee, O God, the Eternal Father, that they do always remember him.” (Doctrine and Covenants 20: 77,79)
As always, this is just the opinion of one Mormon, but I believe that if during the sacrament you consider that you are willing to (in other words, if you want to) take upon yourself the name of Christ and always remember him and keep his commandments, you should witness that to God by taking the sacrament. It has little to do with specific things you have done, and very much to do with what you want to do now and in the future.