But WILL he? Up until now, we've been prompted, encouraged to root for him. We sympathize with him, and have been given every possible reason to look upon Dr. Horrible as the guy to cheer for. Captain Hammer has no redeeming qualities. None at all, according to all the source material available. So clearly, we should be all 'Yeah, Doc's gonna kill Cap, end of story.'
But that's not likely, is it? First of all, Joss Whedon rarely goes for the pat, easy, predictable ending. And second of all, outright murder might just push Dr. Horrible over the line of acceptance from bumbling supervillain to down and dirty bad guy, and in the spirit of this show, will we accept our protagonist as a cold-blooded killer? Sure, Joss has made us root for bad guys before, in the form of Angel and Spike, in the grey-area-morality Mal Reynolds, and so on. But honestly, in the spirit of this show, I suspect we're in for a wide range of possibilities, a few of which follow...
Dr. Horrible killing Captain Hammer: 5-1
Dr. Horrible dying to save Penny after she's caught in the crossfire of the inevitable battle: 3-1.
Penny dying because of the crossfire: 10-1
Captain Hammer turning out to be Bad Horse in disguise: 50-1
Captain Hammer becoming a supervillain: 8-1
Dr. Horrible turning over a new leaf to become a superhero: 15-1
Moist and Penny running off together: 100-1
Dr. Horrible and Captain Hammer falling in love with one another: 1,000-1
Honestly, if anyone dies, it won't be Captain Hammer. By the same token, Penny dying would be cliched, and Dr. Horrible dying would be disappointing. Moist is too minor of a character to matter at this stage of the game, so his death wouldn't carry any real weight. I'm thus stumped as to how exactly this story's going to end... and that's just the way I like it.
BTW: That shot of Dr. Horrible right before he launches into his final song? Abso-freaking-lutely perfect. NPH is superb in that scene.