He doesn't have to know he is lying to lie.
Wasn't there a stunt to oath-bind him against this too? And I suspect that Nilul has a chance of maybe finding it out, even when Ragmar doesn't know himself that he is lying.
Oh I don't doubt he'll tell us the truth, but it's easy to lie with a truth. Sure he'll tell us where Lily's enemy is, but then that enemy will no doubt shift locations or be located in an impenetrable fortress. He'll promise to not harm people and then burn and salt their fields. He'll promise to not act against us and instead merely help our enemies. He'll tell us what is in the mountain and the information will be worthless. He'll promise us faith and serve only betrayal. He is a traitor, he is an enemy, he is going to die.
Give some credit to Uly and Nilul, seriously. While Ragmar is
extremely good at what he does, so are Uly and Nilul, who hold the advantage in this situation. And can be even more so, if The Lily boosts both of them to be sharper on the look-out against the lies. While he has a
chance of betraying us later, which he will likely ty sooner or later, in this immediate situation we have a good chance of getting out extremely important and hard to find information with no traps mixed in. Though he will try it if he thinks he can get away with it and is willing to risk his life, of course.
But remember, he wants to live, which is why he is offering information that won't directly hurt him, but he thinks is valuable to us. If we find out that he is lying, he is nearly guaranteed to die. So again, not screwing us over makes sense if he is unsure about his ability to do so reliably, and can offer information that has no great negative repercussions on him, at least in the short-term.
More relevant arguments against letting him live would be things like that culling him here and now would have a good chance of scattering his beastmen that are still holding some of our land. I'm not sure if the "won't hurt Lusengese ever again" will cover leaving lands he has already conquered and defending himself against invasions in his currently held territory.
Also, the prestige and support we would get from slaying yet an another Anathema, with this being the one who has been hounding Luseng and the Realm for a thousand years. It could go well hand in hand with the themes of the Grand Heresy. And things like sorcerous materials, which have been mentioned before. Lunar heart-essence bathed in lies and thousands of different masks could be very useful for certain Workings.