Thinking on it, I feel the need to expound on what a fucking idiot Tooru actually is.
Bro knows that Sain has at-will mind control and has used it to, among other things, sell the Saint into slavery.
And Tooru is both keeping that info to himself, and plans to just publicly murder Sain with no explanation to his many allies in high places.
Which is both an insane disservice to his allies and just the stupidest possible way of going about it.
When what he should be doing is (1) alerting his high ranking allies that "hey, Sain has at-will mind control, you're in danger when you meet him" and (2) getting it out there that he mind-controls his party members and sold the Saint into slavery.
(They haven't met and appraised him yet, so they don't know it only works on women lower-level than Sain. But that's still most queens and princesses, I would think.)
Tooru could turn Sain into a wanted criminal if he actually bothered to leverage any of the connections he's earned and evidence he's obtained. As opposed to killing Sain out of the blue and turning himself into a pariah.
And there's a work-around for the whole "we need Sain to slay the demon king" issue.
Which is just fucking magically enslave him and force him to carry out his duty. That's absolutely a thing that can be done in this setting.
Tooru doesn't have to murder Sain; he could turn Sain into property with the blessing of the alliance, make him do the thing he was born for, and then legally dispose of him.