Actually they following the Bible.
"If a man is found sleeping with another man's wife, both the man who slept with her and the women MUST DIE" [Deut 22:22]
"If a man commits adultery with another man's wife-- with the wife of his neighbour--both the adulterer and adulteress MUST BE PUT TO DEATH" [Leviticus, 20:10]
Kill Women who are not virgins on their wedding night. (Deut, 22:20-21)
Happy to kill children! Being "Blessed" by kill children.
"Happy shall He be, that taketh and dasheth thy little ones against the stones." [Psalm, 137:9 from Kjv]
"How blessed will be the one who seizes and dashes your little ones against the rock." [Psalm 137:9 Nasb]
"If ur right eye cause u to sin, gouge it out and throw it away. It is better for u to lose one part of ur body than for ur whole body to be thrown into hell" [Matthew 5:29]
"If ur hand sins, it is better to cut off instead of going to hell with both hands" [Mark, 9:43]
Note: In Matthew 5:17-20 Jesus said, he not came for destroy any law but fulfill it and if anyone break only one law and teach people for same to do it then he never can enter the kingdom of heaven so indeed Old Testament also is the law of Christianity.
***Christians follow their Bible or not - this is not a point.