WordNet sense 1 (shockingly brutal or cruel):

flagitious, grievous, heinous, monstrous

WordNet sense 2 (exceptionally bad or displeasing):

abominable, awful, dreadful, painful, terrible, unspeakable

WordNet sense 3 (having undesirable or negative qualities):

bad

WordNet sense 4 (provoking horror):

frightful, horrible, horrifying, ugly

WordNet sense 5 (causing alarm or fear):

alarming