Well, disregarding the implications Bendis did, which were never entirely clear, nor ever followed up on. (that the Void is actually somehow tapping into the power of the Angel of Death)

Otherwise, the Void was actually always the ORIGINAL Bob! The real personality, since Bob was actually a hoodlum, a drug-addict and a thief, and when given such power, he of course became a villain.

But he was still ashamed of himself, so hence, he created another personality as well, that could balance out all the bad things he did - the Sentry.

I do believe this is still the official explanation, hmm...