It is unknown how one must become a Sin Architect, but because of their name, and hints made by Relius and Kokonoe, it can be assumed that all three Architects have at some point made horrendous crimes in the name of science and were present at the birth of Nu -No. 13- and the rebirth of Ignis and Ada Clover. Also, they could have possibly been responsible for both of the aforementioned events.
According to Relius, one of the two remaining Sin Architects, aside from himself, "has thrown away the ability", or so it seems. This is why he no longer has use for them.