Zoro is also the only one that would mentally be prepared to do it. Do you honestly think even if Luffy was on speed dial during that talk, he would have the willingness to commit such a deed? Zoro is the most adult/mature enough one of the group that can put aside personal feelings and get things done. So if it would turn into a showdown, he's the only one that would be up to the task.
It's not always just about strength, yes Zoro is stronger than Sanji, obviously. But not to the extent that wouldn't be one bloody lengthened fight. For him to ask that wasn't just a greater than or equal to comparison.
Again, you can't escape from this
Is literally... literal
Sanji asks Zoro "if I become a super strong but evil mad guy, please kill me"
If Zoro wasn't a fair amount stronger than that hypothetical version of Sanji, that evil Sanji (who would even hurt women, let alone a male comrade) would just kill him in turn, as he wouldn't care at all of him being a comrade
So the thing can only make sense if Pre Power Up Zoro > hypothetical Sanji > Sanji
Otherwise it makes zero sense
It's undesputable
Because Sanji would lose his mind completely, so there's no argument "he would let himself being killed and asks the only person who would be mentally prepared to do that"
No
He wouldn't let himself killed
On the opposite, he would try to kill people around him for the fun of it
Now one can appreciate the meaning of their bond and what not, and I can agree, absolutely
Doesn't change it's something that inherently needs Zoro to be much stronger than Sanji to work
Otherwise is just pure nonsense
As the result would not be the one Sanji wished for, but the exact opposite
I don't think Sanji is that stupid, do you?