Many have pointed the finger at the obvious suspect, her stepson, Tuthmosis III, or possibly his son and successor, Amenhotep II (or even both of them).
Hatshepsut, the dead pharaoh's chief wife and widow, stepped in to help as her stepson's regent but ended up ruling beside him as a fully fledged pharaoh.
After all, if she was really so cold-blooded, she would have found a way to get rid of her stepson since he was the biggest threat to her power, especially after he came of age, but she never did.