Again, we really wouldn't know till afterward. He could by now be little more than a figurehead and senile for all we know, but even that tells us a lot. It's really just like a serial killer or mobster, the police have no idea what they are going to find out till they have them for interrogation. He probably knows a lot that will be useful even if a lot goes stale, a lot might serves as confirmation of this or that other piece of intel, he might just totally clam up or he might crack immediately or he might decide to rave and rant and spew info. Considering the cost and risk of keeping him a prisoner are both fairly negligible, we've nothing to lose and a lot to possibly gain by trying. Even if we don't get all medieval on him we're likely to be able to squeeze a lot of intel out of him, but it's likely a lot of times to be confirmation of other intel, elimination of others, etc. I'm not too sure what you're getting at, honestly. If we get him, milk him till his dry and execute him, seems pretty straight-forward, if he dies during capture or while imprisoned, well, good riddance to bad rubbish.