En fides eius quem patrios Penates secum portare aiunt "Here is the faithfulness of him who they say carried his paternal Penates with him [i.e., Aeneas]." quem is a relative pronoun here. It's accusative in Latin because it's in indirect discourse after aiunt.
Etiam si pugnae fortuna dubia fuisset, quem metui moritura? - She says, "I should have killed them when I had the chance." If there had been a pitched battle between the Carthaginians under Dido and the Trojans, "even if the outcome of the battle had been in doubt, who would I have feared, since I was going to die anyway?" quem here is an interrogative pronoun, accusative because it's the direct object of metui[/i.] She slips from the pluperfect subjunctive [i]fuissetin the protasis of the conditional--normal for a contrary-to-fact condition--into the perfect indicative metui, a more "vivid" mode of expression.