Because one of Agar's ancestors created a gate between the far realms and the prime plane, one that could only be opened with the blood of his line that has been awakened through contact with creatures from the far realms. The portal can only be opened by one of the ancestors line, and only if said family member has had a certain amount of prior contact with the far realms (to provent the portal from being accidently opened and dooming the world). Agar needs to be willing to open it, so they can't just kill him and spill his blood on the portal.
Surely Agar didn't choose to follow this alienist thing all on his own, did he? All sorts of subtle manipulations and suggestions have been floating around his skull since childhood, pushing him into further contact with the realms (left to his own devices, he would have been a baker).
So -
The demon's in question serve a master who draws power from insanity, so setting up a permanent portal slowly increases the creep of insanity in the world.
*or*
The demons serve the elven wizard trapped in the far realms (mentioned in the Manual of the Planes entry). He wants out, and he doesn't really care who he has to hurt to do so.
This, of course, may be too planar-invasiony after the underground lair of the ghouls.
Perhaps Agar's son is meant to be boy-child that the evil fey were waiting for in the timeless fortress? A demon-born child that will take over the fairy realm and turn it into a tangled mess of madness (A new far realms, if you will).