It wouldn't need to break the sound barrier. Killing someone once a day should mean one death every 24 hours (assuming the hand uses its own hand time zone).

The Earth's circumfrence is about 40,000 km, meaning the hand would need to travel at most 20,000 km in 24 hours which requires a speed of 833 km/h, whereas the speed of sound is 1235 km/h.

I suppose if it's the former it would be even safer, as you could somewhat plan to stay out of it's way. Or like, if it's over Europe and you're in Africa, you know you're 100% safe for the next little while.

For the latter, maybe it exists in a dimension above our own. By which I mean... imagine if you were a 2D circle on a piece of paper, with a 3D hand pressing it's finger on the piece of paper. When the hand lifts it's finger off the paper, it effectively vanishes from your field of reference, then when it presses down somewhere else on the paper it looks like it magically came from nowhere.

What if the target person is indoors or underground? Does it just smash through?