Are you sure you aren't just cherry-picking one example of such behavior and applying it to all, just because it would prove your point?
My point is that the belief itself is inert and harmless, and only in conjunction with other desires and beliefs, since, just as you say, anyone who 'merely' believes in the messiah will take the easy out given to them. Lest you object that choosing martydrom must somehow indicate the belief alone is enough, I offer a trenchant example of the curious social dynamics that can drive some rare instances of apparently irrational behavior.
I can imagine one or two lunatics to do such a thing just to make their names immortal
Much more than that. Of the few thousand Christian martyrs during the brief Roman persecutions, how many courted death rather than had it forced on them without any opportunity to engage in a profunctory face-saving gesture?
I haven't heard any examples of people just waltzing into a terrorist camp, declaring their beliefs so they will get killed.
You haven't? You really need to look into this more. For starters: every Christian missionary into North Korea (South Korean or American), and for that matter, South Korean missionaries in general (the 2007 incident in Afghanistan comes to mind). You would have to have a death-wish to sneak into North Korea and try to evangelize!
Even if such examples existed, would you claim that they are the rule, rather than the exception?
I would say martyrs are by far the extreme exceptions among the billions of people who have held the belief in question; when you look at rare outliers, it usually tends to be the case that the causes are idiosyncratic and themselves rare. It's rare to have a death-wish, rare to be so enamored of religious status one will seek death, rare to have schizophrenia, rare to have a mystical experience which unfortunately ends in the perceived necessity to evangelize.
Similarly, when a passenger jetliner crashes in the USA or Western Europe these days, the cause is often something bizarre or rare like Saudi terrorists or a murder-suicide by the copilot (Germanwings isn't even the only such example), and while one couldn't write off all such cases as murder-suicides or Saudi terrorists, one can predict in general that something weird was going on with a crashed flight and that in general flights don't crash. (In case the analogy isn't clear: martyr : normal believer :: murder-suicide-plane-crash : normal successful flight.)
Of course there are less people who were killed for their faith than people who were not killed, I wasn't contesting this. I was only contesting your claim that people who are killed for their faiths don't really believe and are just suicidal. Do you think that all (or most) missionaries who go to a dangerous area, do it with the explicit purpose of getting killed, and do not believe in their cause? This seems to be a common bias, when people think that as they are always right and their ideological opponents are always wrong, their opponents can't possibl...
I am not sure that it is productive to tell certain people that they do not really believe what they claim to believe, and that they only believe they believe it. I have an alternative suggestion that could possibly be more useful.
Binary Beliefs
It seems that human beings have two kinds of beliefs: binary beliefs and quasi-Bayesian beliefs. The binary beliefs are what we usually think of as beliefs, simple statements which are true or false like "Two and two make four," "The sun will rise tomorrow," "The Messiah is coming," and so on.
Binary beliefs are basically voluntary. We can choose such beliefs much as we can choose to lift our arms and legs. If I say "the sun will rise tomorrow," I am choosing to say this, just as I can choose to lift my arm. I can even choose the internal factor. I can choose to say to myself, "the sun will rise tomorrow." And I can also choose to say that the sun will NOT rise. I can choose to say this to others, and I can even choose to say it to myself, within my own head.
Of course, it would be reasonable to respond to this by saying that this does not mean that someone can choose to believe that the sun will not rise. Even if he says this to himself, he still does not act as though the sun is not going to rise. He won't start making preparations for a freezing world, for example. The answer to this is that choosing to believe something is more than choosing to say it to oneself and to others. Rather, it is choosing to conform the whole of one's life to the idea that this is true. And someone could indeed choose to believe that the sun will not rise in this sense, if he thought he had a reason to do so. If he did so choose, he would indeed begin to make preparations for a dark world, because he would be choosing to conform his actions to that opinion. And he would do this voluntarily, just as someone can voluntarily lift his arm.
Quasi-Bayesian Beliefs
At the same time, human beings have quasi-Bayesian beliefs. These are true degrees of belief like probabilities, never really becoming absolutely certain of the truth or falsity of anything, but sometimes coming very close. These are internal estimates of the mind, and are basically non-voluntary. Instead of depending on choice, they actually depend on evidence, although they are influenced by other factors as well. A person cannot choose to increase or decrease this estimate, although he can go and look for evidence. On account of the flawed nature of the mind, if someone only looks for confirming evidence and ignores disconfirming evidence, this estimate in principle can go very high even when the objective state of the evidence does not justify this.
Belief in Belief
It seems to me that what we usually call belief in belief basically means that someone holds a binary belief together with a quasi-Bayesian belief which conflicts with it. So he says "The Messiah is coming," saying it to himself and others, and in every way acting as though this is true, even though his internal Bayesian estimate is that after all these thousands of years, the evidence is strongly against this. So he has a positive binary belief while having a very low estimate of the probability of this belief.
The reason why this often happens with religion in particular is that religious beliefs very often do not have huge negative consequences if they are mistaken. In principle, someone can choose to believe that if he jumps from the window of the tenth story of a building, he will be ok. In practice, no one will choose this on account of his non-voluntary Bayesian estimate that he is very likely to be hurt if does so. But a person does not notice much harm from believing the Messiah is coming, and so he can choose to believe it even if his internal estimate says that it is likely to be false.
A cautionary note: one might be tempted to think that religious people in general have belief in belief in this sense, that they all really know that their religions are unlikely to be true. This is not the case. There are plenty of ways to distort the internal estimate, even though one cannot directly choose this estimate. I know many very religious people who clearly have an extremely high internal estimate of the truth of their religion. They REALLY BELIEVE it is true, in the fullest possible sense. But on the other hand I also know others, also extremely devout, who clearly have an internal estimate which is extremely low: they are virtually certain that their religion is false, and yet in every way, externally and internally, they act and think as though it were true.