Because Bill was taken by several people, not one. So, we need to use the plural form: "ones who took Bill".

Ones= Several individuals that make up a whole sample population/body. : One of a whole. 1+1+1+1+1+1+...+n= 100% [several ones of a whole] Literally, one (1) is one of the ones. If n/n=1 1 of 100% or 1/n is one of a whole or one (1 part) of the ones (the variable units that make the whole population of objects or persons).

why there is an "s " behind the one?: "He's one of the ones who took Bill." WHY IS THERE an "s" AT THE END OF THE WORD "one"? Evgenius' answer is correct. So we could write the sentence this way: "He's one of the people (or persons) who took Bill."