in the standard decomposition of a simple Lie algebra

the subalgebras

are, of course, nilpotent, so the corresponding matrices in the adjoint representation are nilpotent. As TS correctly says, a Killing norm of a nilpotent generator is zero.
Here there is no contradiction with the non-degeneracy of the Killing form, because this is a bilinear form on the complex algebra, not a hermitian form. E.g., if

and

are orthonormal vectors in a real vector space, then

.
One should NOT say that

belong to the real algebra

. They live in the complex algebra

.