alamolgerecht

We may not see and understand it as good does not mean that its not good.
To your other question, why don’t you have this issue with every aveirah that anything can not take place without the rotzon haboreh including this persons aveira, and hashem himself is bound to the torah… look at rashi what sholosh shutfim beodom means and also take time to understand what the issur is by your case of mamzer and with all the above your answer is self understood.