Kyran said what he said because he knew Osric was up to no good.
If Osric convinced Light that Kyran and Haylee might be at odds, but only the former was hostile towards the latter, Light might side with Osric to detain Kyran.
Of course, this was only Kyran's guess, and Osric might have a different plan in mind, but that was unimportant. What mattered to Kyran was preventing Light and Osric from working together.
Though Light was inside Kyran's Void domain, it was impossible to imprison the former for long. The Void domain only prevented all mages inside from using magic or circulating their magic energies. However, it did not stop people inside from moving. The domain was also immobile; thus, if Light decided to leave, it was not that hard to do so.
