Itaque, ut aliqua in uita formido improbis esset posita, apud inferos eius modi quaedam illi antiqui supplicia impiis constituta esse uoluerunt, quod uidelicet intellegebant, eis remotis, non esse mortem ipsam pertimescendam.
aliqua is nominative and modifies formido
uoluerunt – see Lewis and Short uolo B 8:
8 To be of opinion that something is or was, = censere, dicere, but implying that the opinion is erroneous or doubtful, usu. in the third pers., sometimes in the second.
(a) To imagine, consider: est genus hominum qui esse se primos omnium rerum volunt, Nec sunt, Ter. Eun. 2, 2, 17: semper auget adsentator id quod is cujus ad voluntatem dicitur vult esse magnum, Cic. Lael. 26, 98: si quis patricius, si quis—quod illi volunt invidiosius esse—Claudius diceret, Liv. 6, 40, 13.—
(b) To be of opinion, to hold: vultis, opinor, nihil esse … in naturā praeter ignem, Cic. N. D. 3, 14, 36: volunt illi omnes … eādem condicione nasci, id. Div. 2, 44, 93: vultis evenire omnia fato, id. ib. 2, 9, 24: alteri censent, etc., alteri volunt a rebus fatum omne relegari, id. Fat. 19, 45: vultis a dis immortalibus hominibus dispertiri somnia, id. N. D. 3, 39, 93; id. Tusc. 1, 10, 20; id. Fin. 3, 11, 36; id. Rep. 2, 26, 48: volunt quidam … iram in pectore moveri effervescente circa cor sanguine, Sen. Ira, 2, 19, 3.—
(c) To say, assert: si tam familiaris erat Clodiae quam tu esse vis, as you say he is, Cic. Cael. 21, 53: sit sane tanta quanta tu illam esse vis, id. Or. 1, 55, 23: ad pastum et ad procreandi voluptatem hoc divinum animal procreatum esse voluerunt: quo nihil mihi videtur esse absurdius, id. Fin. 2, 13, 40; 2, 17, 55; 2, 42, 131; 2, 46, 142; id. Fat. 18, 41.—With perf. inf.: Rhodi ego non fui: me vult fuisse, Cic. Planc. 34, 84.—
(d) To pretend, with perf. inf., both subjects denoting the same person: unde homines dum se falso terrore coacti Effugisse volunt, etc., Lucr. 3, 69 (cf. A. 1. n. supra). (ε) To mean, with perf. inf.: utrum scientem vultis contra foedera fecisse, an inscientem? Cic. Balb. 5, 13.— With pres. inf.: quam primum istud, quod esse vis? what do you mean by as soon as possible? Sen. Ep. 117, 24.—(ζ) Rarely in the first pers., implying that the opinion is open to discussion: ut et mihi, quae ego vellem non esse oratoris, concederes, what according to my opinion is not the orator’s province, Cic. Or. 1, 17, 74.
http://perseus.uchicago.edu/cgi-bin/philologic/getobject.pl?c.20:1257.lewisandshort
quaedam modifies supplicia, and eis refers anaphorically to supplicia.
eis remotis – ablative absolute: “these having been removed,” “if these were to be removed,” i.e., “without these”.
And thus, so that some fear should be held out in front of the wicked [i.e., so that the wicked should be deterred by fear from practicing wickedness], people in earlier times used to claim that certain/some punishments of this sort had been set up among the denizens of the underworld [apud inferos], because, of course, they understood that without them death itself wasn’t something to be afraid of."
uidelicet is probably ironical. Cicero contemptuously rejected the idea of punishment in the afterlife in the Tusculan Disputations, as probably most educated Romans would have.