Killing splits the soul, right. Using whatever spell then puts that piece of soul into an object, creating a Horcrux. I think it was Slughorn who said that no man ever split his soul more than once, other than Voldemort. But how? I understand he was talking about Horcruxes, but that's merely the act of saving that lost piece of soul. Doesn't killing more than one person split your soul more than once? If not, how did Voldemort do it? Shouldn't anyone who has killed seven or more people look like Voldemort then?
He meant that the Dark Lord was the only one to split his soul to create multiple Horcruxes.
Although he refers to it as "splitting the soul" it's implicit that he means splitting the soul, to then turn the split portion into a Horcrux. It's also very heavily implied that using a murder to split the soul and creating a Horcrux with it is far more damaging than simply committing murder without creating a Horcrux. This may be because even after committing a murder and splitting the soul, without creating a Horcrux, all the pieces of soul stay in the person they belong to. Whatever the reason, it's clear that creating a Horcrux is a step beyond murder on the scale of evil.
“By an act of evil – the supreme act of evil. By committing murder. Killing rips the soul apart. The wizard intent upon creating a Horcrux would use the damage to his advantage: he would encase the torn portion –’ ‘Encase? But how –?” - Harry Potter and the Half-Blood Prince, Chapter 23 (Horcruxes)
Most wizards who commit murder likely don't do it with the intention of creating a Horcrux, and it's implied that every mention of soul splitting is specifically as it applies to Horcrux creation rather than simply murder.
“You heard Voldemort: what he particularly wanted from Horace was an opinion on what would happen to the wizard who created more than one Horcrux, what would happen to the wizard so determined to evade death that he would be prepared to murder many times, rip his soul repeatedly, so as to store it in many, separately concealed Horcruxes. No book would have given him that information. As far as I know – as far, I am sure, as Voldemort knew – no wizard had ever done more than tear his soul in two.” - Harry Potter and the Half-Blood Prince, Chapter 23 (Horcruxes)
It becomes a bit more clear that they're talking about multiple Horcruxes and not simply multiple murders (without using the murder to create Horcruxes) when Dumbledore explains how he came to suspect that the Dark Lord had multiple Horcruxes. Here, it's fairly clear that he means "splitting the soul multiple times to encase the pieces in multiple objects creating Horcruxes" as the actions that put the Dark Lord beyond the realms of usual evil.
“I, who have gone further than anybody along the path that leads to immortality.” That was what you told me he said. “Further than anybody.” And I thought I knew what that meant, though the Death Eaters did not. He was referring to his Horcruxes, Horcruxes in the plural, Harry, which I do not believe any other wizard has ever had. Yet it fitted: Lord Voldemort had seemed to grow less human with the passing years, and the transformation he had undergone seemed to me to be only explicable if his soul was mutilated beyond the realms of what we might call usual evil …” - Harry Potter and the Half-Blood Prince, Chapter 23 (Horcruxes)
The piece of soul has to be deliberately intended to be split off, with the intent of creating a Horcrux. Presumably just killing doesn't split the soul - otherwise everyone's souls would be ripped up, even Molly Weasley, and Voldemort's soul only split while killing Harry because it was already so weak after being split so many times.
"I do think that the strict definition of "Horcrux," once I write the [Harry Potter Encyclopedia], will have to be given, and that the definition will be that a receptacle is prepared by Dark Magic to become the receptacle of a fragmented piece of soul, and that that piece of soul was deliberately detached from the master soul to act as a future safeguard, or anchor, to life, and a safeguard against death."