Yeah that is something we never really touched upon. I got a nice little idea tho. Instead of some evil entity, the guy who did it is actually a generally good guy. He cares for his people, he follows orders, however, he follows Gold.
Gold is an asshole, but not everyone knows that. Everyone thinks he's a great and kind leader. Kinda like Homelander, a shining hero to everyone BUT his enemies, who are few of them because they keep dying. Gold Leaf knows he's an asshole, so Gold sends a hit squad.
In Stella's view, the Metals who did that were hulking giant monsters that destroyed her house and "killed" her which in turn killed her dad. Every flashback with them is nightmarish and horrifying, but that's because it was with 8 year old Stella's perspective.
To an 8 year old, EVERYONE is a giant.
The Metal who actually did it? Neither of them recognized the other when they met. Stella's older, with a new perspective. She meets a stainless steel, a swashbuckling charming bad bot that bends the rules and crosses the line of being a troublemaker, but still a noble hero. Problem is, he's still an alien.
Much like the gems, he doesn't comprehend humans. He didn't know Stella was a kid, he didn't know GL could become an important human organ, he wasn't there. To Stella, it was a horrible and tragic day that changed her life forever.
To Stainless Steel and the hit squad, it was the equivalent of the ATF shooting the home owners dog.>Hey that's his kid>What's a kid?>using these human clip art images, I believe that a kid is Earth slang for a "pet">well you know the rules, pets gotta go in case they attack us defending the owner>do we have to? It's kinda adorable >yes>Oh well. Nothing personnel, "kid">*SNKKT*