Now the Spirit of the LORD had departed from Saul, and an evil spirit from the LORD tormented him. (1 Samuel 16:14)
Clearly, it was a literal evil spirit.
Again here, that is not necessarily the case. There is a reason why there are so many religions coming form this book who all say different things and that reason is because everybody tries to interpret all these scriptures you've used here different ways and insist the others are wrong.
The word translated as, "spirit" in the Bible does not always mean literally an angel. It also means breath, wind, and mental disposition. Bible scholars look for consistency and frequency when they look at the Bible, especially with scriptures like this one. So looking at this what we have is a scripture stating that God sent an evil spirit to torment somebody. Yet elsewhere in the bible we find, "With evil things God cannot be tried nor does he himself try anyone." Now set aside any preconcieved notions from ANYONE ELSE'S interpretation of this scripture and just use the Bible by itself.
This spirit cannot be literal because God has no dealings with evil people or disobedient people. This is proven by the very context of the verse at hand, what had happened was Saul disobeyed, as he often did. Samuel had just told him, "To obey is better than a sacrifice" because Saul just didn't get it. He thought too much of himself to the point where he did what he wanted isntead of what he was told to do. This is precicely what causes literal evil spirits, as in the case of the demons who decided to forsake their proper place in heaven to take human wives for themselves in Noahs day. God would never use a literal evil spirit to accomplish anything, he has no use for such ones and has demonstrated this all through the bible from beginning to end in every single other occasion. Thus this one example cannot be a literal creature, it would not be consistent with the rest of the Bible.
What makes far more sense in this instance, and is consistent with the bible, is for this spirit to be a mental disposition instead of a creature. Further I would submit that it wasn't actually SENT by god but rather ALLOWED by him. As an example you could think of Pharoah of Egypt in the bible. God is spoken of as, "Making his heart hard" or some verbiage as that. But that cannot be literal for the same reason this instance with samuel cannot be, rather God fooled them much the same as the prophet in your original post, by allowing their own minds to do the work for him. He need not literally make Pharaoh's heart hard, his pride would do it for him as he was made the fool of by the plagues. The same is true of Samuel and his egotism, he had done wrong and he knew it sure as he knew it was wrong before he ever did it. This made his conscience torment him, caused him to be afraid God would punish him somehow. Another person you could think of is Job, Job saw his problems as coming form God when in fact they did not - he merely allowed them. Much the same it's very likely Samuel believed that Sauls mental anguish was from God so he wrote it that way when in fact it wasn't, or it's possible that it's a translation error being that the bible is so old. But regardless you can rest assured that God has no dealings with evil beings, thus he did not send an evil creature to do anything at any time ever, nor will he ever do so.
Having said that, as someone else here said you can pick apart the bible and find things to contend with. But the fact is it very much is consistent when taken all together. It is easy to become angry with God for allowing what is happening to happen, and when you want to be angry it's easy to find reasons to be angry like these. I have spent many years angry with God myself. But the fact is that everything we need is in that book. We are directed by it not to argue over it's meaning but to follow it's direction, as paul said not to be involved with, "debates of words." The fact is we have an idea of what to expect in the future according to the bible. Whether you interpret it as a future on the earth or in heaven is irrelevent, because according to paul there are no more prophets to tell us what the prophecies mean. But we don't need to know that anyway, we were directed to keep on the watch by Jesus as a secondary thing; the primary commandment of Christ being love. The whole point is to be loving, be modest, be faithul and have hope. Do not argue, do not debate, do not divide, do not do the very things religion does today. Nobody focuses on, "the sort of person they ought to be" when that's the only thing that matters for us.
Regardless of what the future will be, when the end of NOW arrives and you tell Jesus, "Look! I god this right, and that right, I was wrong here and here, but i guessed so many things accurately and I spent so much time figuring out the past..." what is he going to say to that person? He's going to say, "See I was hungry and you did not feed me, see I was thirsty and you gave me nothing to drink, see i was naked and you did not clothe me." Regardless of what you think might happen, or what you think happened, if you focus on the sort of person you ought to be and follow the commandment of Christ that's all that matters. "To obey is better than a sacrifice." (1 Samuel 15:22)