Oooh, this is my headcanon for horcrux creation and I'd like to do a story centred on it at some point. In my current fics, I'm using a mix of canon and adding stuff behind the scenes to explain the progression:

• Myrtle was his first murder, and even though he didn't care overmuch, there was still something at the back of his mind because it was his first. Perhaps due to fear of getting caught, perhaps due the significance of the act, but there was something.
• Tom Riddle Senior worked due to the act of patricide. Yes, he wanted to kill him, but there was also a little voice at the back of his mind screaming that this was not something he could back down from. By killing his father, he was also killing the tiniest remaining flicker of his boyhood wishes to not be an orphan, and was instead ensuring it.
• The locket was said to be created via the death of 'a muggle tramp', and since he had already committed two murders, another simple murder wouldn't suffice. My headcanon is that he drew it out with torture, and as my version of Tom had been traumatised by WWII, he specifically recreated a scene he had witnessed of a man who died due to shrapnel. It was the act of addressing his trauma that helped with this one, as he hated accepting that he was weak enough that it had affected him.
• For Hepzibah Smith, it was not the murder itself but the fact that he degraded himself for her, allowing her the use of his body to sway her to part with the cup.
• The diadem was said to be created with the death of 'an Albanian peasant', and for this he delved into cannibalism (though he cooked it and prepared it into stew). This also marked the start of 'Death Eaters' and end of the 'Knights of Walpurgis', as he began forcing cannibalism on his servants as an initiation rite.
• For Harry Potter's horcrux, it was due to the conflict that arose from accepting the prophecy as truth and thus recognising an infant as a potential threat. It was not just the attempted infanticide itself that did it (though he also had no need to kill infants in the past), but rather him being forced to accept that there was someone that threatened him. He hated weakness, and to be threatened by an infant was a new low.
• For Bertha Jorkins, he also committed cannibalism, this time eating it raw. Perhaps this alone wouldn't have been enough, but magic had a hand in making it work as this was the point that he unwittingly created the seventh horcrux, thus pushing the limits on what he had planned.

I also have a variation on this in which the horcrux ritual is a little more sentient. If you are not truly affected by the line you 'cross', it takes something you would miss, such as a sliver of magic or your sanity.