a depraved wretch
The phrasing suggests not just a vile being, as "wretch" was used in I:4:3 and I:4:6, but now also one who is corrupt, wicked. Victor, who has no discernible religious belief or instruction, has wandered into a treacherous theological morass. The Judeo-Christian God has created humanity, in Milton's words, "just and right,/ Sufficient to have stood, though free to fall" (Paradise Lost, III.98-99). Victor, however, as God's stand-in, has no assurance that his Creature is without flaw. Having constructed him from whatever bodily parts he could lay his hands on (and without any particular nicety that they even be human), Victor presumes a correlation between body and soul, both being corrupt. In theological terms, however, such a vision of God as the creator of evil is fundamentally heretical, for it would hold God directly responsible for human depravity. Victor's chain of thoughts does not extend so far as openly to convict himself of the responsibility for the evil he assigns his Creature. Yet, his sudden sense here of their twin relationship does suggest the glimmer of that terrible truth the novel will slowly unfold.
Johnson's 1755 Dictionary represents the verb to deprave with an uncharacteristic lack of discrimination:
To vitiate; to corrupt; to contaminate.
The Oxford English Dictionary, on the other hand, registers historical and theological shifts in usage:
depravity An extension of pravity (ad. L. pravitas) previously used in same sense, after deprave and its derivatives. (No corresponding form in Latin or French.) The quality or condition of being depraved or corrupt.
a. Perverted or corrupted quality. Obs.
b. Perversion of the moral faculties; corruption, viciousness, abandoned wickedness.
c. Theol. The innate corruption of human nature due to original sin. Often total depravity: In common use from the time of Jonathan Edwards: the earlier terms were pravity and depravation.
d. A depraved act or practice.
1. Rendered bad or worse; perverted, vitiated, debased, corrupt. Now chiefly of taste, appetite, and the like.
2. spec. Rendered morally bad; corrupt; wicked.