If I use the "no offense" in a sentence, it is usually stated as "no offense intended". I would be commenting on a situation rather than directing it at someone in particular. Anyone who uses "but" like this is totally negating the "no offense" anyway. A bit like saying "I like you but", the word "but" cancels out the preceding statement. So you are saying, "I don't like you, here's why". So many people use the English language incorrectly, me included, no wonder foreigners have such a hard time grasping it, english speaking people can't even grasp it.