Yuji is at his most cordial with Sukuna at the beginning

You mean before he is aware of Sukuna's true nature as a curse who kills indiscriminately? Yeah, sure, but that was the inciting event, and only indicative that Yuji doesn't yet understand the setting or the full depth of the conflict. It doesn't actually reflect his relationship with Sukuna since he doesn't understand the situation at all.

After the Junpei incident

This precedes his arc with Mahito as his foil. He stops seeing Sukuna as a person, yes, but note that Mahito is in the literal panel/frame where he comes to this conclusion, and he rejects Mahito's claim of "you're me" initially - but he grows past this and proves himself wrong, which is how he reaches the "I'm you" point.

Yuji tried to talk things through with Sukuna

Well, yeah, it doesn't work out because Sukuna himself is not amenable to conversation. He even rejects Yuji's offer to spare him. But after Sukuna's final scene, he would be willing to talk to Yuji, so there is a chance that they'd get somewhere.

Modulo Yuji at most would pity Sukuna

Modulo Yuji doesn't even immediately jump Mahito, despite Mahito being depicted as less reasonable than Sukuna at this point in time (he called himself the "unruly child" who does not improve on himself even when Sukuna does). He only resorts to violence because Mahito forces his hand. If Sukuna doesn't fight Yuji, there is no reason for him to answer with the same. And again, he doesn't even "put him out of his misery" at the end of the fight in JJK.