Let A be a HTM & L(A)={m/m accepts w}
As we do not know the behaviour of m, we have to consider all possibility.
if m accepts w, then A will accept m & will go to accept state.
If m rejects w, then A will reject w & will go to reject state.
if m goes to looping, A will neither go to accept nor to reject state.
But as A is a HTM it must end up either in accept or reject. It means our contradiction is wrong that A is HTM. It means m cannot have a HTM. So for m, membership is undecidable.