Compassion isn't even a factor -- self-interest is. Do you have any idea how many pureblood wizards you were responsible for killing? The fewer there are, the more inbred they will become, and the more they will need to interbreed with muggles. Not only that, but with fewer wizards, there will be less goods and services available to you from wizarding society. You plan to be immortal, do you want your immortal life to be spent surrounded by steadily more inbred wizards? With people who become more stupid and weak with every passing generation?
You can be a sociopath and still find use in people, and still have good reasons not to kill people that aren't rooted in fear of punishment.
It simply requires intelligence to accompany your sociopathy, enough intelligence to examine the second and third order consequences of your actions.
But Lord Voldemort was an idiot who was defeated five times by a child.