Even though Sun had surrendered and had a link with him without resisting and was even visibly happy getting along with Thorn now, Luciel knew that he might still cause some trouble later.
Luciel already saw that as long as his life isn't threatened he could be very bold.
And such a personality is hard to change even after clearing up the misconceptions it had with humans.
In his past life, he had disciplined the plants that he had many times because after some time passed they would forget what happened to them and try to cause trouble again.
So, with Sun's personality that he had seen, after he forgot what happened to him after some time passed, he would probably cause trouble.
Of course, whether the trouble is big or small is still unknown.
If they're just harmless pranks Luciel doesn't mind it as long as they're not too frequent, but if it caused a bigger problem then it needed to be disciplined.
