His parents being criminal doesnt change the fact that they are female lycans, aka useful for the lycans.
And they erased the part of his memory about him being a female lycan and changed it to make him think he was a male. He had already marked Lee Hwa so if they tried that on the kid, they could have tried it on the parents.
And even after the uke knew he was a female, still marked to Lee Hwa and even pregnant with Lee Hwa, the lycans still believes they can use him as their female. (It was even brought by the future heir that he could take the uke by force if needed). So it's still quite stupid they killed the parents.

Ok but wtf, why did the lycans kill his parents and not the kid????? Bruh
And why kill the parents at all??? Female lycans are rare asf, and you just killed 2 of em??? The fuck bitch, you psychos and stupids.