Frank J. Wilstach, comp. A Dictionary of Similes. 1916.

Merciless

Merciless as a male tiger.
—Anonymous

Merciless as Cæsar.
—Anonymous

Merciless as the grave.
—Anonymous

Merciless … as that of trying to read the Universal Riddle.
—Lafcadio Hearn

Merciless as ambition.
—Joseph Joubert

Merciless as Othello.
—Ouida

Merciless as waste desire.
—John Payne