it's funny that sonic sees good in metal sonic and thinks he can become good when like the very first thing that he had seen metal sonic doing is metal sonic beating up innocent animals as shown with the metal sonic holograms at sonic cd and how metal sonic keeps trying to kill him every damn time in various ways
sonic doesn't have rose tinted lens where red flags are regular flags to him: he sees metal sonic with multiple red flags and a big glowing neon sign directly pointed at him with words that say "HE CANNOT BE FIXED" which sonic's all "i want him and can fix him!" at metal sonic
i think the whole "sonic sees good in metal sonic" plot point came from idw, not from the games, but i could be wrong. i don't really read or interact with idw that much save for a few arcs here and there since i don't really... like the characterization or writing of amy, sonic, and metal sonic altogether, but i won't really shit on anyone who does love idw!! it's just not my cup of tea.
i do agree that sonic doesn't have rose tinted glasses LOL. sonic knows full well what he's going into, he understands that metal sonic is incredibly dangerous and has hurt people before and may continue to hurt others, but that's the fun part—err not the hurting others part but the fact that sonic can just go on adventures, find this guy terrorizing people, and end up going on a journey to stop him. in a way, it's fun being able to go against metal sonic himself, and if he's able to save people along the way, that's a cool bonus to it!
i don't think he'd want to fix metal sonic in the "i know you're good" kind of way; i think he'd want to fix metal sonic in the "you also deserve freedom way because i know you're capable of it; i've literally seen it before" way. he understand that this guy runs on pure anger and hate towards him, and he's willing to take that in stride. what he wants for metal sonic is for him to have the option to do things for himself instead of just being eggman's pet especially since he's known to be rebellious but constantly gets lobotomized in canon just to keep him at bay.
tl;dr: sonic would see the bad in metal sonic and think, "that guy is unfixable, but i think a little dose of being able to choose for himself is gonna do him wonders."
btw i hope this doesn't sound rude or anything i'm worried my tone comes across as too deadpan at times especially when i'm yapping like crazy!!!