Was it us policy to kill smart slaves?

No. There was never a US Government policy to kill smart slaves.

There is, however, no doubt that many slave owners believed "a literate slave was a dangerous slave" and would stifle attempts to educate slaves, through litigation and/or intimidation.

Starting in the mid 1700's, several states passed laws prohibiting slave education.

While these laws typically did not clarify any consequences for the slaves who might attain literacy, the financial consequences for teachers were clear.

See Narrative of the Life of Frederick Douglass an American Slave, written by Frederick Douglass, for further information.