Exactly what the title says, Christianity in all its various strains still stems from the horrendous old testament, and with all the awful shit that endorses, as well as with things like Calvinism and the general doomsday cult foundations in the religion, why do some people still act so charitably towards it? Christianity has proven itself to be a morally corrupt and oppressive institution at every turn. Or maybe I'm just too negative? 
EDIT Ok guys I get it this was rather reductionist, and I definitely was disregarding all the good that religious folks have done, and that religion has inspired people to do, but again over correcting into enthusiastic support for religion can also be problematic.
Pretty much all religions provide a basis for a social belief system/set of principals and morals by which a society should be and behave. They often also provide the meaning to life. Organised religions, e.g. Christianity, then provide a regular means of contact to teach this, e.g. via church services, for people. Many people who need structure and security, or the answer to why they exist, seek this out. Others engage with it as a point of ego boosting (e.g. social figurehead within the religion) or power (leader) or money (albeit moreso historically). In summary it provides individuals with a major part of their life, livelihood and welbeing.
So, when these people learn of all the horrendous crap associated with THEIR religion, they either cannot, or refuse to, accept it as their religion teaches the perfect way to be and therefore cannot possibly be fallible. Or they're culpable in the deceit, abuse and hypocrasy. Or they can't face the idea of loosing their religion (and therefore life as they know it/want it) so choose to lie that everything is ok and good. Everyone who disagrees or opposes is a heathen and must be shut down or worse, killed.
To add, as they replied as I was typing; Infamousblt makes the good point that there are still good people within these societies, which is true of pretty much all societal groups; there are good and bad people in all of them.