Your take is interesting though I thought up something a little different. In my fan-fiction continuation I had Rat King wanting to show Shredder what he is capable of after "Wrath of Rat King" and team up with Baxter the Fly and Razhar and Tokka. Shredder in a previous script I did used Razhar and Tokka when he tells them he is their "father" and they want revenge on him like the other two. In the end Baxter and Z are out of the fight and so are Razhar and Tokka and the two villians have to work together. Rat King later was the field leader in a second grouping of rogues.
Its unfortuante the '87 series never gave reasoning for the Rat King's motives or any orgin. All we have so far is Tales of TMNT vol.2#35.
