I mean he does like Kael that's for sure but he also wishes for both Kael and Eunkyeol to be at peace. Kael had his time and he wasted it by being too taken up by grief that he made innocent people suffer for it. He saw in the early chapter that the Kael he once knew and loved was "corrupted", I don't think he'll give Talon hell for trying to do it.
He said he wanted to HELP Kael to be released from the curse, not kill him. Being at peace doesn't mean he has to die? Talon came up with the solution, telling Sajun that he will be saved if Sajun completed the curse by pouring his mana and wishes into the curse. And might I remind you again that Eunkyeol, although his soul is practically the same, is a different consciousness from Kael. And Eunkyeol has been suffering his whole life without knowing why. Will Sajun just stay still knowing Talon intends to kill him? Will he not 'give him hell' knowing an innocent person has to die without even knowing anything?
Sajun has the plot armor so I'm not that worried. But Talon said smth about getting rid of BOTH the demon and Kael since the curse is almost complete. Was that his plan from the beginning? If it really was, then I can imagine Sajun's reaction after realizing how he took a part in a plan that involves killing Kael and I'm afraid that would drive a wedge between him and Talon