Tchar may go on to further corrupt the Chainbreakers once he's annuled Ghreer's specific deal and countless others in faustian bargains like Angron had been told it would do by every person he's heard of chaos from, but at least we'd be safe from the moral inequity of *checks notes* guillotining a aspect of one of the most horrible tyrants the galaxy has ever known.
Look, i'm just going to be blunt here. As much as people are going on about how this will surely taint the new god, violence is not always unjustified and mercy in the face of injustice is not always righteous. Protecting horrible people in such a way that it leaves them capable of continuing to prey on new victims is not some wholesome chungus move with no possible negative implications, it is Bad Actually in a way that i'll eat my hat if it would not taint this god in a way we'd abhor.
And let's be real here, that whats them freeing him is. This is no meaningful possibility of reforming Tchar, and enslaving is something Angron rightfully considers morally abhorrent even if we know it would horribly backfire from Malcador. Unlike when dealing with a rl person, Tchar's nature as a Daemon make stopping him any other way short of a imprisonment so draconian that it is effectively torture and arguably a fate worse then death impossible.
If you have qualms with the idea of ANgron killing someone no longer capble of defending themselves...
I'm just going to go out on a limb here and say that Angron is 100% down to kill awful people who are absolutely defenseless before him and incapable of resisting, even if those people used to nominally be comrades with him. Mostly because that was what the Kill Kurze vote was all about, and back then people argued we should let the monster go out of nebulous high minded ideal that will absolutely cause them to bring untold suffering on countless more people.
We made the right choice then. Let's make it again here.
[X] Slay Tchar
P.S. As for the arguement about how 'They were a really good ally of coneivence to us and always did right by Angron so who cares how our actions might impact other people, even ones we love'...
"I'm going to spare him solely because you've personally benefited me regardless of the evil you've done, will do, and intend to do in the future' is actually a really fucked up stance.
If your ally is a horrible person who will continue to do horrible things to people, it's bad NOT to betray them and let them get off scot free for the things they've done regardless of how nice they've been to you personally. "Don't overlook other people awfulness because they were nice to you personally' is one of the most basic tenets of fairness and justice anyone is ever taught.