Who do you think would hate Thad and who do you think would be in his corner in the flash family if he were to ever return?
I feel like it's always a guarantee that Wally and iris would hate him but what about the rest? Who would want to throw him to the curb and who would want to keep him around to try and help him?
HATE as in "We don't kill, but for HIM that is an exception" - Iris 100%. The only thing Iris has ever felt for that boy is pure loathing and disgust and had zero remorse after learning what Wally did to him. If she wasn't absolutely terrified of changing history or altering the timeline she probably would have found a way to kill him in the tank before he was even born.
EXTREME dislike, hate, loathing, will NOT talk to or be friendly with in the slightest it is a FIGHT on sight - Jay, Wally and Bart. They won't murder him, but they will turn him into a living statue and break his legs before using him as a projectile weapon and not feel bad about it at all. But they won't kill him. They don't want to open their homes to him, they don't want to forgive him. The threshold for talking is done the second Thad actually succeeded in killing Bart. It's done.
Dislike, but is willing to open a dialogue - Max. He doesn't trust him and is extremely cautious of him. He tried hard to get him to abandon his quest for murder in the Speed Force, but there was no future Thad wanted with Max that also had Bart in it. Max knows what he is capable of, what he wants, and what he WILL do. Thad saw the alternative life he could have had and denied it. Still... Max is old, has seen things and done things himself, and while he is not a perfect man or a perfect mentor for teens, he knows when someone has been damaged so he's willing to talk to him if necessary.
Cautious, but willing to try to help him - Barry. Everyone hates him for it but Barry doesn't have any solid emotional ties to Bart (despite what Impulse 1995 implied and that is a "do with what you will" scenario), so knowing what he did doesn't impact him as much as the others. I know you hated reading that, but it is fact. He loves Bart but he just doesn't KNOW him or have a real bond with him. He's dealt with clones killing loves ones before, he's murdered before as a response to it, but Thad is a child soldier. This is going into headcanon territory but it wouldn't be unbelievable for Barry to believe that he is partially responsible for the feud the Thawne line carried into the 30th century because he murdered Eobard - thus he helped create Thad in the first place, and was unintentionally at fault for the murder of his own children. It's unfair and a lot to put on that blonde man but it's not unbelievable. So he takes on Thad, to redeem him, to redeem himself, because it might be the "right" thing to do for a person that had clearly been created without love or even a care for his autonomy. Everyone is unhappy, Iris is betrayed, Bart feels even more betrayed because why does his murderer get to bond more with Barry than he ever did? 🙃
Bonus
Total oblivious but takes the kid in and only puts two and two together years later and by then it's too late - John Fox. Look, he does have records of history but it's still pretty spotty. But he found this gremlin somewhere in the 4700s stranded and offered to take him home to the 853rd century and Thad just.... stayed there. It's pretty quiet and it's not at all bad being one of three speedsters and no one is trying to kill you on sight. When John finds out it's been years and he hasn't DONE anything other than snap at Impulse One Million, who snapped back at him. <- entirely headcanon
Note: this is for the COMICS and the comics alone, no other form of media. This is also taking into consideration current canon with FMA and Thad's ultimate goal in Mercury Falling - to kill Bart, specifically.
Note two: If anyone wanted a feel good ending for Thad, please check out my Ao3 where there are some Thad fics exploring him taking Max's offer and abandoning his quest.
