Okay, once on SC2 forums I heard a theory that Xel'naga created zerg and protoss so that they would get reborn when these two races would merge, since one of them contains essence of purity, and other ssence of life or something else I don't remember. So see, the perfect, 50/50% combination of Protoss of zerg would be a Xel'naga reborn, while the hybrids are more perverted impure versions that seek to break the cycle of Xel'naga by destroying the zerg and protoss that are ready to get fused together to true Xel'naga... also that guy stated that Tassadar may be the one of protoss who's ready to do that, since he didn't died and someway got preserved after his death...