After reading this chapter, I don't think Strike really killed Shiro's mother in the way he described. Strike sounds like a coward from what I read, he probably became afraid of the monster and Shiro's mother died trying to save his sissyass. He then proceeded to tell her daughter that he had killed her mother as a form of atonement. My reasoning is as follows, most reincarnators are people who either lived unfortunate lives or died in horrible accidents. Not a single one so far seemed to be a bad person, which implies that the God or Goddess who revived them in that world choses who is worthy of living there.
It would be strange for a scumbag like Strike to be chosen and given that much power if he was in fact scum. Therefore I find it more believable that he is simply a coward and that his atitude is a form of self inflicted punishment.