Board Thread:Warhammer 40k General Discussion/@comment-14745711-20140911102114/@comment-25177957-20140914203914

Still, no details were given about a family. If he did belong to a family ever, he was probably within several hundred families throughout his life since it is said he fathered many children and had many wives. (Although no one is sure what happened to the male children he fathered).

Even if the warp could send something back in time. It wouldn't work if they sent Horus all the way back to when the Emperor was born, since most of the Chaos gods weren't born when he was, their power might not be able to follow Horus, and Horus would be unbuffed by Chaos. And it's likely the corruption and psychological effects of Chaos on his mind would be lessened. Also I think the warp would have to spill out more into realspace and have greater influence on realspace time in order for extra long range time travel to be possible.

To bring up another point in that the Chaos Gods are not to be taken for fools, they knew that without the Anathema, Mankind would have died out long ago and been unable to feed Chaos. So that could explain why they didn't think up a plan to kill the Emperor as a child.