Absolutely wrong!  Where, O where, did you learn your Thomism?  From where do you think the Church draws its greatly important distinction between mortal sin and venial sin?  From the SUBJECTIVE!  And where do you think THAT came from, from among many others, including Thomas.  With all respect, you need to read him before you reject him.  

BTW:  "There MAY be sinfulness as well" in the Holocaust?  Do you seriously contend that the Holocaust was "a terrible wrong," but not objectively so?