qui mori timore nisi ego

The storyteller Niceros asks a rhetorical question: what sort of person could die from fright if not himself?

In this construction, a form of possum in the subjunctive appears to have undergone ellipsis.  The construction likely a relative clause of characteristic.  A relative clause in the indicative would state something as fact, whereas a relative clause of characteristic conveys a sense of potential (A&G 534)—given that Niceros’ statement is rhetorical, a subjunctive mood is more fitting than the indicative.

qui: there is no antecedent for the relative pronoun qui; where the antecedent is undefined, a relative clause with the subjunctive indicates a characteristic of the antecedent (A&G 535).

mori: the present active infinitive of the deponent verb morior; here, a complementary infinitive to the absent form of possum.

timore: ablative of cause (A&G 404a).

nisi: although often used with conditions, nisi may be used by comic poets in the sense of only when a negative is expressed or understood in the main clause (A&G 525e).  There is no main clause preceding, but there is an understood negative: Niceros implies that no one could be more afraid than he.  Translate here as “save” or “except”.

ego: Niceros emphasizes his exceptional state of fright.