Christian point of view : God was bored and needed a set of doormats/housepets that he could use from time to time as a punching bag, so he created Man, but He gave him free will so he would transgress and could then be punished! And transgress he did...so He cast man aside so He could gleefully watch him suffer for his disobedience -- and yea suffer man did, but he didnt do quite as badly as God expected him to, and as years went by, Man evolved, and he actually had a LOT of fun creating obstacles for himself and then overcoming them, and worse, man forgot that he was being punished ---- and WORST OF ALL, man all but forgot that he was initially created by a bored God to worship Him!
So He decided to remind Man that - he, as in, man had a superior being who was watching him, and that Man was being punished and that he better make it look like he's suffering, AND that the only way out of his punishment (should he eventually realize that he was being punished) was to accept His Son as the messiah, etc. Well "some" took it to heart and as for the others, they carried on like nothing ever happened.
So the "believers" combined their forces and waged war on the "unbelievers" and massacred many of them so that they could save their heathen souls from the horrible punishment of life on earth followed by afterlife in the eternal pit of fire ---- and the list of "believers" grew.... and the story rolls on!