In the time Lyric and Elizabeth have been missing, they were in the news often, there were vigils, fundraisers, etc. The killer had to have seen that these girls were people, not objects, IMO.
Someone capable of abducting and killing Lyric and Lizzie probably doesn't have the ability to be filled with guilt - if so, I doubt he would have remained silent for five months. JMO.
But I sure hope I'm wrong . . .
He might confess after being arrested, if it benefits him to do so.
Hopefully he/they will feel guilty, but they may just feel oddly proud of the sense of power they have over the family, the community and LE.
They may even find it hard to resist bragging about it to someone, somewhere, sometime...