What did Reverse do to Zinfandel brother to cause his death?
I'll say sorry in advance if the answer is a bit confusing but I'll try and explain it the best I can. Also, Zinfandel and Coretto belong to @stankychee but she said it was okay for me to talk a little about this part of Zin's background.
Reverse kidnapped Corretto because he looked like a Papyrus and was very easy prey; young, a bit frail-looking, and friendly enough to approach and lure closer. But Zinfandel and Corretto are from a Swap AU (Outer!Fellswap Gold with a dash of Warhammer 40k inspiration) and that made Corretto a Sans-coded monster instead of having a Papyrus-coded soul, something Reverse was not aware of at the time.
You see, to fix Ghost's (Reverse/Rewind's brother) broken soul, Reverse needs a Papyrus-coded soul to fill in the missing data and also give Ghost a new body that is attuned to the new coding. It can only be a Papyrus-coded soul and body... Nothing else.
So after successfully kidnapping Corretto, Reverse tried to combine Corretto's soul with Ghost's and it failed horribly because the souls weren't compatible with each other. The failure, along with Reverse's stubbornness and denial, resulted in Corretto's death and Ghost's soul getting damaged even further. It was not a merciful death for the younger monster...
Zinfandel is currently hunting Reverse down and chasing after him through the multiverse to avenge his little brother.