If The Toad hates rats and wants to kill them all then why does he have Spike and Whitey who are rats working for him?.

the toad uses them to catch Roddy and Rita.

I think because he knows that they are not smart.

He's using them. He probably plans to kill them AFTER they've helped him kill all the other rats in the world.

I don't know why I'm talking in the present tense. :P