Abe Masatoyo served as a retainer of the Matsudaira of Mikawa Province. He was the eldest son and heir of Abe Sadayoshi.
In 1535, Masatoyo and Sadayoshi joined Matsudaira Kiyoyasu, lord of Okazaki Castle in Mikawa, in an attack on Moriyama Castle in Owari Province. The castle was defended by Oda Nobumitsu, the younger brother of Oda Nobuhide. The action intended to address the threat of attacks on the Matsudaira by the Oda. In the midst of this battle, a rumor circulated that Masatoyo’s father, Sadayoshi, was colluding with Oda Nobuhide to launch a rebellion. Sadayoshi informed Masatoyo that Kiyoyasu may purge him, and entrusted Masayoshi with a letter proclaiming his innocence. On 12/5 of 1535, after hearing a disturbance by the horses in the camp, Masatoyo mistakenly concluded that Kiyoyasu had punished his father, whereupon he promptly cut-off Kiyoyasu’s head near the entrance to the castle. Masatoyo was in turn immediately slayed by Uemura Ujiaki. This incident is known as the Collapse at Moriyama (Moriyama kuzure). Kiyoyasu perished at the young age of twenty-five. Upon hearing of the act committed by Masatoyo, Sadayoshi attempted to kill himself, but was forgiven by Matsudaira Hirotada, Kiyoyasu’s son and heir.
The sword used by Masatoyo was known as a Sengo Muramasa from Kuwana in Ise Province, a location known for manufacturing high-quality swords. This type of sword was used widely by bushō from Mikawa including Tokugawa Ieyasu.