Why is Kazuma mom even atoning for what he did, he is a "killer", but he disposed off a trash that psychically abused her daughter, raped her and caused to commit suicide.
She's raising Shiika as a parent she lost, but in 7 years she should have noticed or known father most likely also tried to sexually abuse her or read her sadness as loss of her parent. Still it's a long time to not know at least pieces of the life Shiika had.
Shiika visiting Yuri grave and praying at the altar might be to let Kazuma guard down, but don't think that's the case.
Kazuma suicide would cause his mother to go into deeper despair, so would death from being killed by vengeance. He is in emotional turmoil, so can understand he isn't thinking straight...
Thanks for scanlation!