I think a quote from an answer to a different question might also add something here:
JKR: I see it as a series of things you would have to do. So you would have to perform a spell. But you would also - I don't even know if I want to say it out loud, I know that sounds funny. But I did really think it through. There are two things that I think are too horrible, actually, to go into detail about. One of them is how Pettigrew brought Voldemort back into a rudimentary body. 'Cause I told my editor what I thought happened there, and she looked as though she was gonna vomit. And then - and the other thing is, how you make a Horcrux. And I don't even like - I don't know. Will it be in the Encyclopedia? I don't know if I can bring myself to, ummm ... I don't know.
This quote is from an Interview with Jo Rowling which was originally brought to my attention by Richard when he was answering the linked question.
My point here is, obviously, the process of making a Horcrux is not just more than murder (and murder is obviously to be differentiated from killing, as well), it is magnificently horrible.
Now obviously Tom Riddle wasn't afraid of getting his hands dirty, but one gathers that making a Horcrux is an involved process and the fact that he killed many on his rise to power is only one part of the puzzle.
It is also worth remembering that he needs things to make into Horcruxes:
'And they could be anything?' said Harry. 'They could be old tin cans, or, I dunno, empty potion bottles ...?'
'You are thinking of Portkeys, Harry, which must be ordinary objects, easy to overlook. But Lord Voldemort use tin cans or old potion bottles to guard his own precious soul?'
Harry Potter and the Half-Blood Prince - p.471 - Bloomsbury - Chapter 33, Horcruxes
On the whole, though, these are minor additional points. I think the main points are his fixation on the number 7 and the danger of having more than one Horcrux, alluded to elsewhere.
'And the more I've read about them,' said Hermione, 'the more horrible they seem, and the less I can believe that he actually made six. It warns in this book how unstable you make the rest of your soul by ripping it, and that's just by making one Horcrux!'
Harry Potter and the Deathly Hallows - p.89 - Bloomsbury - Chapter 6, The Ghoul in Pyjamas
Let's not forget, the whole reason he had the tête-à-tête with Slughorn was because he wanted an opinion on what would happen if he went for the seven-part soul.