They all did their best--to kill being the chiefest ambition of the human race and the earliest incident in its history--but only the Christian civilization has scored a triumph to be proud of. Two or three centuries from now it will be recognized that all the competent killers are Christians; then the pagan world will go to school to the Christian--not to acquire his religion, but his guns. The Turk and the Chinaman will buy those to kill missionaries and converts with."
Of course, Twain knows nothing of Hitler or Stalin or ISIS. He knows nothing of the terrible suffering that has been wrought in the name of Nothing and Allah. Regardless, it is not religion at fault here, but religious people, trusting in religion and religious things to be their salvation. It is sinners, with sinful natures, doing sinful things, in a sinful world. What other result could one expect?