Voldemort's goals were mainly Grindenwald's taken to a bigger extreme. Where Grindenwald wanted Wizards to rule muggles "for their own good" and assert Wizard superiority, Voldemort wanted to purge muggles (SPECIALLY muggle-born wizards) to assert the same superiority.

However while Grindenwald could have maybe, just maybe, sincerely believed in his own "for their own good" talk, Voldemort is simply obsessed with the idea that Wizards are superior to muggles, and should be treated like gods by them. When he met Dumbledore for the first time, he firmly believed that a wizard was even beyond death itself, that his father was the wizard, because a wizard couldn't die like his mother.

Both stem from the same idea that wizards are superior to muggles and deserved to be treated as such, but Grindenwald was capable of feeling remorse and empathy and regret, while Voldemort never learned these things. There was nothing in the way between him and his cruelty.

His obsession with finding immortality comes partly from his conviction that a wizard, or at least only he should be above everything, even Death itself. Is it a god complex? Yeah pretty much, but a god complex is just a superiority complex taken to extremes, and Grindenwald had that.

And from this obsession with immortality comes his obsession with Harry. For him, Harry means his death. Even with all those Horcruxes he knew that if it wasn't Harry, it would be him.

I personally think that if he was adopted by a loving muggle family when young, he'd be just a normal wizard that speaks parseltongue. Or he could be the one that finds the true, benign secret to eternal life, I dunno.