Choshu Domain had their branch, Chofu clan take care and protect Tadamitsu, but during the Kinmon incident in 1864, in Shimonoseki War, when the conventional party within the domain gained power due to the First Conquest of Choshu, he was assassinated in Tako Village, Toyoura District of Chofu Domain in the December of the same year he was kept as a refuge.
長州藩は忠光の身柄を支藩の長府藩に預けて保護したが、元治元年（1864年）の禁門の変、下関戦争、第一次長州征伐によって藩内俗論派が台頭すると、潜居中の同年11月に長府藩の豊浦郡田耕村で暗殺された。