because usa forced elections to palestine assembly in 2006 despite objections of israel and pa government (which was afraid that hamas that will win).

when hamas won, usa was horrified by outcome and "sponsored" PA security forces to get rid of hamas in gaza, but hamas prevailed and killed everybody who were against (throwing from buildings, dragging behind bikes) it or tortured them into submission

Because Hamas gleefully kills anyone who opposes them.