Well, you see, Satsuki gets raped by her mom in the bath.

So in revenge, because she's too badass to be mindbroken ahegao sex-addicted, she stabs her mom in the back and toss her onto a cross with three spikes, crucifying her and literally making a shower of blood. Pretty fun.

Now I'll never take any Satsuki doujins seriously, because the truth is known.