You're forgetting the counterfactual. Namely, that we live in The Best Of All Possible Worlds and what you describe as horror is actually the nicest things can conceivably get. The standard Christian argument is that, without God, existence would be significantly worse. Also (depending on your flavor of Christianity) the mortal life is a proving ground not a final destination. Life is a trial one experiences before being eligible to enter the Kingdom Of Heaven, where God is fully in control.

The horrors are a consequence of Free Will mixed with the corruptive influences of evil spirits sent out to tempt mortals to sin. And they are transient, while the Christian Reward is supposed to be eternal. You see this best in the Story of Job, during which he suffers a litany of torments but holds firm to his faith. This faith is ultimately rewarded, not just through the restoration of his material pleasures, but through the promise of an eternal blissful afterlife.