Someone or something that's contemptible receives harsh judgment and deserves it. Your desire to bring to justice the contemptible coward who stole your last chocolate bar seemed noble, until you realized you'd eaten it the night before.
Contemptible means "worthy of contempt," and is one of several words of its kind that end with -ible rather than -able, which would mean something more like "capable of contempt," (and isn't a word). You could remember that “I find it contemptible!” begins with “I.” If you don't, some will consider your spelling errors to be minor offenses, while others will declare them . . . contemptible!