I haven't read all of the replies, so this may have been said already, but Voldemort was basically a genocidal sociopath bent on world domination and immortality.
He despised muggles, even though he was 'half-muggle' (called muggle born in the books), and wanted to kill every last one of them. He was an evil sociopath from the very beginning, evidenced by the fact that at ten years old he had already murdered a fellow orphan's pet rabbit and had 'done something' to another two that left them permanently scarred, mentally/emotionally if not physically.
And he tried for immortality because he was terrified of death.