The reason behind the Third Secret being horrendous is that it fits in with the descriptions of the coming Apocalypse.

This entails a devastation on all fronts, famine, pestilence and war, however it goes deeper than that in so far as the spiritual implications of chaos and ultimate subjugation under a perceived 'saviour' of mankind in the darkest moments.

Your senses are grossly overwhelmed by these events to the point of desperation and you become swept up in the deception so as to actively contribute. You are lost at this point and will fall into the narrative as if your life depends upon it... guess what!