Not "who" but "what" ? This is a big big subject, Timmy.
It was the same thing that has killed everybody who has ever died, from Adam and Eve till now.
CONDEMNATION is the killer. There is no condemnation in Christ Jesus. (
Rom 8:1)
The children of Israel murmured, complained and transgressed all the way to Mt Sinai,
but nobody died, because they had Christ with them.
At the Red Sea Moses' rod was a type of the cross of Christ. He stood up and held that rod out over the sea and said, "Stand still and see Jeshuah [Jesus]." (
Ex 14:13)
At Marah, Moses threw a tree into the bitter water, and it was made sweet; there's the cross of Christ. (
Ex 15:24)
At the Wilderness of Sin, (
Ex 16:8) He gave them manna. Jesus is the bread that came down from heaven.
At Rephidim, Moses smote the rock and they drank (
Ex 17:3) from, "....that spiritual rock that followed them, and that rock was Christ."
Then after the Law was given in
Ex chapter 20, sin revived and they died, (
Rom 7:9) by the millions. (
II Cor 3:7), calls the giving of the law: "the ministration of death". In (vs:9) Paul calls it "the ministration of CONDEMNATION"
Even those who were bitten by the fiery serpents lived if they only looked at the brass snake that Moses made and put up on a pole. That snake was a type of Christ.