He had spent three years in the navy following a childhood reportedly marred by his father’s suicide and his mother’s alleged neglect and devotion to church activities.
Details of his upbringing have stoked anger in Japan against the Unification Church and garnered Yamagami sympathy, with supporters showing support for him through donations and a petition calling for leniency.
The Unification Church was founded in Korea in the 1950s by self-styled messiah Sun Myung Moon.
In a letter published by Japanese media, Yamagami accused Abe of supporting the sect and expressed resentment towards the group.
The church has confirmed his mother’s membership but refused to specify the amount of donations she made, which reports said may have totalled around ¥100 million (US$700,000).
Less than a year after Abe’s death, in April, a man hurled an explosive device towards Japanese Prime Minister Fumio Kishida shortly before he was due to deliver a campaign speech in the city of Wakayama.
The leader escaped unharmed, but the fact that an assailant was able to throw the device at such close range prompted renewed criticism of security arrangements in Japan.