I was just now reading his wiki article, CW: suicide

spoiler

after his dad committed suicide when Yamagami was 4 years old

his mom joined the Moonies and gave away 720,000 USD worth of assets she inherited from her father, including his childhood home. He couldn't attend university due to the resulting poverty and due to nearsightedness he couldn't become a firefighter either and went to vocational school with his paternal uncle's financial support instead. That same uncle basically made his family's survival possible, paying for their food and living expenses repeatedly after the mom impoverished them. Apparently he once threw a cup of tea in her face when she asked for Unification Church donations in lieu of support for her kids.

CW: suicide, again

spoiler

Eventually one of his brothers lost eyesight in an eye due to an untreated illness, that went untreated due to lack of funding (aka directly due to the cult) and killed himself. Yamagami ultimately tried and failed to kill himself also so that his military life insurance could go to pay for his neglected younger brother's living expenses.

Just absolutely awful from beginning to end, I feel so sorry for this guy. Good on him for being the change he wanted to see in the world.