Is it illegal for the attorney in fact under a power of attorney to remove funds from the estate after grandmother's death without the consent of the executors?

Yes. The power of attorney for an attorney in fact is extinguished upon the death of the principal. Therefore, the power of attorney was no longer in effect at all after your grandmother's death. The attorney-in-fact has no authority to remove funds with or without the executor's consent.

An AIF who used their authority under a POA after the death of the principal is stealing.