I doubt that humans set out to create sentient beings. I suspect it was an accident, in an effort to create a smarter machine, they created a fully sentient one. In the beginning they ignored the warning signs. Noone expected their "toaster" to wake up one day and ask, "is there more to life than this?" We have not been given much history on this subject. By the time the humans understood what happened, the war may have already started. To extrapolate further, it is possible that the mechnical Cylons attempted to improve themselves, accidentially creating a better model that in turn rejected their creators. Perhaps the cycle of time is happening at both a macro and micro level.