Fate/Zero

When Kiritsugu was a child, he lived on Alimango Island in the Philippines with his father Noritaka Emiya and his father's assistant, a local girl named Shirley whom Kiritsugu was smitten with. Noritaka conducted research on immortality using plants, which had Shirley convinced that this would one day change the world, as she hoped that Kiritsugu would follow in his father's footsteps. Before leaving one morning, Noritaka tells Kiritsugu to stay in the house, after the former believes that someone went into his lab the previous night. However, Kiritsugu went out to search for Shirley instead, only to find her at her house in a frenzy, devouring live chickens. Shirley revealed that she went into the lab and used the immortality formula on herself so that people could appreciate Noritaka, which turned her into a Dead Apostle, a vampire caused by magecraft. Shirley asked Kiritsugu to kill her before she could infect others, but unable to betray her he instead fled to alert the Church, allowing her to infect the entire village. As chaos spread across the island, the Church Executors and the Mage's Association Enforcers arrived to stop the outbreak and cover up the incident. Kiritsugu was saved by a mercenary named Natalia Kaminski, whose mission was to eliminate the magus responsible for the outbreak. Kiritsugu confronted Noritaka over his work and its consequences, but the latter simply shrugged them off as an accident and planned to continue his experiments elsewhere, leading Kiritsugu to kill Noritaka as a result. Emotionally broken and with nowhere to go, Kiritsugu agreed to escape with Natalia, his ideologies having taken shape.