Thoughts on Thistle's relationship with Eodio (headcanon+analysis)
It's very easy to assume Thistle never got along with Eodio, since the most prominent interaction between them we see is their confrontation, and i do see that sentiment a lot. But I think it's quite underlooked just how present Thistle was in his life.
I think it's safe to say Thistle did have a good relationship with Eodio at some point, we see them sitting merrily at the dinner table, Eodio having a jovial conversation with his father, Thistle smiling fondly at him. And when Eodio held Yaad for the first time, Thistle stood closer to him than even his own wife. i don't think Thistle was totally distant just yet.
My personal headcanon is that for a long time, Thistle genuinely loved and cared about Eodio, just as much as his parents! Practically viewed him as his own child, even. Which made it so frustrating for him to have so much pressure put on him, being treated like he's not doing enough, when he's doing everything he possibly can. He wants Eodio to recover just as much as Delgal, he really does!
But it's just so hard, and he feels like they don't understand just how painful it is for him to do everything in his power to help and never succeed, how terrible it feels watching Eodio be in so much pain and not being able to do anything about it, all while Delgal is being so short with him, expecting more than Thistle can provide because he's stressed. they're all stressed. Eodio is dying and the kingdom is under attack and he's trying he's trying he's trying so hard but nothing is working.
Which makes it even more hurtful to him to watch Eodio grow up and take his recovery for granted. To be ungrateful, when Thistle suffered and struggled so much to keep him alive.
It's not so bad when he's young, of course, a child isn't going to be the best at expressing gratitude. But when Eodio is an adult and still doesn't seem to realize just how much Thistle has done for him, that (in Thistle's mind) he owes him his life, it sets him off. The boy who he loved so much and did everything in his power to save is now yelling in his face, vilifying him, spitting on everything he's done for him...
...And Thistle is not going to tolerate it.
It's a moment of weakness. And at first, he's really only intending for it to be temporary. In his eyes, Eodio is being a spoiled brat, and he's just putting him in time-out. Giving him some time to cool down and think about what he's done, that's all.
...But he never does bring him back. Maybe he just forgot, maybe he decided it wasn't safe, we'll never really know. It's a shame they could never mend it.
I think the animosity Eodio had toward Thistle grew fairly late in his life, some time after Yaad was born. He doesn't seem the type to seethe for years upon years without speaking up.
All that said... I really do wish we saw more of their interactions before their confrontation, how Thistle felt about him... I think people would appreciate Eodio much more if we knew... well, anything about him, really! Bit of a shame we don't.