Many Marvel villains have incalculable body counts. They've slaughtered everyone in front of them, not letting anything stop them. The reasoning isn't always the same, but the results are heaps of bodies littering their path.
Galactus isn't technically a villain, but try telling to that a planet in his path. Galactus has to devour worlds to survive, and many of those worlds were inhabited. The universe has existed for 13.7 billion years, meaning the amount of life lost to Galactus is incalculable.
Even taking into account Thanos killing half the universe in one go, Galactus still wins on kill count. Although not every world he destroyed was inhabited or had a large population, the sheer volume of them in 13.7 billion years is unknowably vast. Scientists on Earth calculate that the amount of dead humans equals those alive, and that's only after a hundred thousand years of human existence. Galactus has had way more time, so he's easily eclipsed Thanos's Gauntlet-powered snap.