I oppose: That person murdered the PCs parents. Maybe the PC forgives him, but his brothers and sister? Radagos sold the kids into slavery (imagine what could have happened to the kids in that time)! It's ok not to execute him, but recruiting him would be a slap into the face of the kids.
Maybe with a choice: You can have either Radagos or your younger silbings in your clan, if you recruit him, they join other clans.
Or timed: He can be recruited until the first of the children becomes 18. Then he disappaers because he knows she hates him and he doesnt't want to wake up one morning discovering she has sliced his throat in the night...