I'm not a teacher, but I would say that you are unlikely to find any rule that explains the presence of doubled consonants. "Assassin" derives ultimately from "hashish" apparently,
Quote:
a. F. assassin, or ad. It. assassino: cf. also Pr. assassin, Pg. assassino, Sp. asesino, med.L. assass nus (OF. forms were assacin, asescin, asisim, hasisin, hassissin, haussasin, etc.; med.L. (pl.) assessini, ascisini, etc.), ad. Arab.  ashsh shn and ashshiyyn, pl. of ashshsh and ashshiyy, lit. ‘a hashish-eater, one addicted to hashish, |
but the real issue here is simply that English orthography is inconsistent and hampered by a character set that is inadequate for the sounds it is used to represent. Also historical changes such as the Great Vowel Shift have given us all sorts of challenging spellings which basically must be learned by rote.