He actually only kills six of the eight people on his list--one has a heart attack, and one is poisoned by someone else.

And, naturally, he gets arrested for the one he didn't kill. He has a line about it in one of the songs: "I've dispatched half a dozen/each one a cousin,/all of them quite by design./This is not a defense,/it just makes no sense!/This murder was not one of mine!"