Some years ago I lived next to a college which trained missionaries for overseas work and I got to know some of those studying there.
Without exception they were incredibly nice people; drawn by a vocation and fired by a fervent if naive view that they were going out into the world to do good.
Some even accepted that they were embarking on a life of risk, and that there was a chance, albeit slim, that they may have to suffer for their faith and possibly even die (from disease and as collateral damage in internecine struggles and feuds if not directly targeted as Christians).
Their conviction and their faith were, possibly, little different to those that drive a suicide bomber, although naturally the would-be missionaries would recoil in horror at the thought of any physical coercion of converts (unlike organisations like the New Tribes Mission, which has perpetrated some foul deeds in the Amazon Basin).
To the college's credit, anyone accepted for missionary training had to have a skill which would demonstrably benefit the communities to which they'd be travelling - resulting in a lot of medics, engineers and teachers passing through.
To its discredit, however, was the evangelical ethos - the assumption, as has been mentioned earlier, of the superiority of their belief systems.
But maybe that's my problem - I have nothing against the propagation of essential truths and the expansion of the Englightenment, but I'm only interested in facts, science and rational humanitarianism. For someone to try to propagate a 'faith'; a core of beliefs which cannot be proven and which will not demonstrably improve the lot of the converted is, to me, pernicious.
By all means go overseas to teach, to cure and to build, but please don't ram an alien belief system down people's throats.
Maybe any religion that feels the need to proseletyse can't be too confident about itself. If your belief system is any good, surely the world will beat a path to your door to adopt it.
But maybe that's the rational humanist in me talking and trolling. As the saying goes, some of my best friends are Christians :^)