This is heading into the realm of fanfic (or fan theory, or something), but this first occured to me when we see the red petals at the bottom of Arthur's house. Unless I'm terribly mistaken (and I might be), they were NEVER explained.
What I think could have happened is that Arthur ate the Red Flowers out of desperation (considering Igor and others have gotten ahold of Red Flowers as well, it's not unfeasable to think he got some), and King was forced to kill him. One of the reasons I think King would've done it is because, well, it's safe to assume he learned how to fight from Arthur. Furthermore, he shares a nearly brother-like relationship with Toroko - he's mad with her at first, but is willing to trade Sue to the Doctor for her return. He could be trying to make up for having to defeat his master.
If you want to add a bit of awesomeness to the theory, King's eye's are red because they are stained with the blood of Arthur when he slayed him. :P
I know it sounds iffy, but it's possible. o_o
