I guess it's because the other 3 had some sympathy points going for them that Toffee didn't:
Ludo was a canon cripple with self-esteem issues who was abused as a child.
Heinous was also abused, and also had issues, and was presented as the poster child for racemixing (also she's female and female villains tend to die less).
Mina is also female and was a crazy war vet who's seen some shit and had her body and mind altered in ways she didn't seem aware would happen when originally signing up. Also she's portrayed as incredibly loyal to her master, and that tends to inspire sympathy even if it's for an evil purpose
Everything about Toffee portrays him as a "privileged" (spellbook claims he was a prince; yeah so was Ludo but we don't see Toffee getting beaten up) sadistic asshole who was doing the "right" thing for more selfish reasons than Star ultimately did. Also he's a canon handsome, possibly wealthy, able-bodied cis male.
But also I think the show at the time he died was just a different product than it ended up being by S4. Maybe Mina or Heinous could've gotten a worse end if this was an earlier season and the show wasn't quite as focused on race relations as it is now. I think Ludo was always going to be redeemed though, or at least by the time they had Toffee use him twice in a row.
It's really odd Mina didn't die though. I feel like removing her powers isn't enough.