D&D 5E Sleep spell overpowered?

[UngeheuerLich]

Sleep can kill 4 kobolds on average with no save

Burning hands can kill any number of kobolds with no save (since even the damage on a succesful save kills a kobold)

Which is the over powered spell?

Range and area is smaller. Also average is only 10.5 which is in 40% of all cases too low to kill a kobold on a successfl safe.
You are also comparing apples and oranges. Both spells have their uses and I have seen both rated good and poorly... which means, that it most probably it depends on the situation.
Magic missile is not bad either, because it kills 1 kobold no matter what happens at a good range without friendly fire... and if you decide to play a bit more risky, it may kill one healthy and one already injured one.
Spells may not be judged in a vacuum. Last weer I learned, that the poor Melf´s acid arrow is better than I thought. It compares well enough against magic missile. Especially against a spellcaster who wants to concentrate on a spell and who can cast shield which would block magic missile.