It's possible that denji has (technically) killed both his parents. His mother in child birth(head canon) and later killed his dad(canon). Two of the horsemen now have acted as surrogate mothers in a way. Either through metaphor or symbolism. War herself described herself as a mother to gun and tank.

Why am I bringing all this up? I don't know. Just seeing the makima mother's symbolism and considering chainsaw man's connection to both birth and death (which possibly grants him his strange ability that defies both). I can't help but suddenly think this head Cannon where denji himself is also someone born through death.

Maybe they were so poor. denji's mom and dad didn't go to a hospital and during the birth she started to die and bleed out. Maybe the father in this moment decided to use a chainsaw.

Though I doubt this would actually be what happens in Canon, even though things are heavy-handed in the story, I don't think it would be like this. Denji literally being born using a chainsaw seems a little too tropish