It was The Hand of God, symbolically. It really doesn’t matter who is going to write “Heisenberg” on the wall at Walter White’s house. “The writing on the wall” or “Mene Mene” is an idiom for “imminent doom or misfortune” and for “the future is predetermined”.
In the book of Daniel, chapter 5, a disembodied hand appeared and wrote on the palace wall the words, “Mene, Mene, Tekel, Upharsin” which means “To count, to count, to weigh, to divide (Persia).” The prophet Daniel was summoned and interpreted this message as the imminent end for the
Babylonian kingdom because of his love of riches. That night, the Babylonian king, Belshazzar, was killed and the Persians sacked the capital city.
Walter White aka Heisenberg is gaining money and power at the cost of dividing his family and destroying his own life.