Those are some interesting questions...

I don't know how anyone could possibly know whether it's true that "everyone likes to fantasize about their own demise". My gut feeling is that most people wonder what death is like, whether they will see that famous "tunnel of light", what's on the other side (if anything), and so forth. But I don't think that most people without a mental illness think about death-by-suicide more than once in a blue moon. They're too busy living. (I could be wrong, of course; that's just my impression.)

I think that suicidal thoughts are pathologized for two reasons. First, they tend to indicate the possible presence of real pathology, that is, depression.

Second, however, is the fact that suicidal thoughts are a cultural taboo in most Western societies. One must not go public with suicidal thoughts, just as one must not go public with thoughts of violence or thoughts of incest, for example. I believe that this is because open acknowledgement that one is having any of these kinds of thoughts tends to frighten other people. I suspect this fear is partly practical and realistic (that is, people don't want suicide or violence or incest to occur for very good reasons), but also is partly a function of the way it reminds the listener that he or she too has had taboo thoughts or desires. People tend to feel guilt and shame about their own taboo thoughts, so we have an implicit social contract not to mention such thoughts to each other. Or so I believe.

As for suicidal ideation and thoughts about suicide, as far as I know they are the same thing.

Thanks for raising some thought-provoking questions!

BlueOak