I have called folks 'nuts' on numerous occasions. I have called other people other somewhat offensive things, too. I don't think name-calling is necessarily a terrible thing, particularly when you're campaigning or agitating for some view or other. It's just part and parcel of political fights where people are more robust in their rhetoric than they would be if they talked to their loved ones. All you indicate is the strength of your deep felt disagreement with someone who you think doesn't deserve much respect with regard to certain views they old.
Usually when I use such terminology I mean to indicate just that strength of my disagreement with the person I refer to as 'nuts'. I only deploy it in cases where the person I call 'nuts' says something particularly implausible, irrational, unreasonable, name it. I'm sure you have heard other people calling someone 'nuts' or 'crazy' with similar intentions.
Who is it that I do not have in mind, when I refer to someone else as 'nuts'? I do not even think of someone with a mental disability. I would never refer to a person with a mental illness as 'nuts'. That would be quite pointlessly offensive and uncalled for.
A couple of weeks ago, in passing, I referred on this blog to a religious fundamentalist as 'nuts' (hint: he would object to that description :) who criticized me harshly in a newspaper article and blog entry, calling me - ironically - 'unhinged'. He made demonstrably false claims about the impact Catholic doctrine has on the clinical care patients can reasonably expect in some Catholic hospitals. I meant to offend this guy. He made obviously false factual statements and I called him on that. I was pretty angry about the audacity this guy displayed when he made these false claims and I wanted to ensure that my readers really understand the depth of my anger. So I called him 'nuts'.
Obviously, I did not call him 'nuts' in the sense that I meant to suggest that he is suffering from a mental illness that rendered him incompetent. Far from it, I very much meant to hold him responsible for the false claims he made, and I had no intention of 'letting him get away' under the pretext that he was not mentally competent anyway. So, calling him 'nuts' simply meant to suggest that his views were unreasonable, irrational, demonstrably false, name it. I can't imagine that any of my readers would have thought that I meant to suggest that the guy I criticized had a mental illness.
And yet, the blog entry caught the attention of a number of people in the US who are involved with groups looking after the legitimate interests of people suffering from mental illnesses. One of them attacked me on this blog pretty vigorously, stressing that my wording is offensive to mentally ill people, because 'nuts' is a term that is used in a derogatory manner by other folks referring to mentally ill people. The writer even suggested that stuff like my blog entry could lead to lynch mobs and other such nasty things. His comments were clearly way over the top, but the fundamental point he made, namely that I had made an error in judgment when I picked a term that is used by some people to offensively refer to mentally ill people, seemed pretty sound to me. I immediately apologized on my blog. It's all nice and well that I did not intend to offend mentally ill people, and that I didn't not even have mentally ill people in mind when I picked the term I chose to offend the religious fundamentalist, but at the end of the day we should take into considerations - as much as is feasible - what the unintended consequences of our rhetoric might be.
Lesson learned, make sure that if you wish to offend 'A', you offend 'A' and only 'A' (or someone who is with regard to a relevant feature like 'A'), and not a whole bunch of other people.