TL;DR: Did Snape ever commit murder? I would guess probably. Is there any canon verification of Snape having committed an outright homicide, excluding Dumbledore's murder? Not that I'm aware of (As always, please let me know if I'm wrong.). My longer answer follows.
"Murder" and "homicide" are technical terms which simply mean one human being causes the death of another. It could be accidental or premeditated, or perhaps even assisted suicide might technically qualify, but clinically, murder is simply the act of taking another's life.
In the US, many states have a felony murder law, which holds a person who is associated with a murder equally as responsible as the murderer. And please note I'm not suggesting there is for certain any kind of law similar to this in the Wizarding world -- we have no idea about that. I'm just using this as an example of what a law addressing certain kinds of murder might look like. I am not saying this is from canon or is canon.
The rule of felony murder is a legal doctrine in some common law jurisdictions that broadens the crime of murder in two ways. First, when an offender kills accidentally or without specific intent to kill in the commission of a felony, the offender can be charged with murder. Second, it makes any participant in such a felony criminally liable for any deaths that occur during or in furtherance of that felony. WIKIPEDIA: FELONY MURDER
I believe it is impossible to discount Snape's killing of Dumbledore in Half-Blood Prince. Snape killed Dumbledore and this is a huge big deal that we can't just exclude. It shows his ability to commit the action, motivations and altruism aside. Why this is important and telling goes back to Goblet of Fire, to a bit of information given out by Mad-Eye Moody (Barty Crouch Jr).
‘Avada Kedavra’s a curse that needs a powerful bit of magic behind it – you could all get your wands out now and point them at me and say the words, and I doubt I’d get so much as a
nose-bleed. But that doesn’t matter. I’m not here to teach you how to do it.
Goblet of Fire - page 192 - Bloomsbury - chapter 14, The Unforgivable Curses
Mad-Eye Moody (Barty Crouch Jr) suggests that Avada Kedavra is not a curse that is easily performed on the first try. Practice was necessary. Snape had enough practice with Avada Kedavra -- whether or human, animal, or magical creature victims -- to be able to successfully murder Dumbledore even though he clearly didn't want to do so. And, as Bellatrix Lestrange says in Order of the Phoenix, a person has to really want to perform an Unforgivable in order to do it successfully. Snape was able to compartmentalize his caring and companionship to Dumbledore well enough to be able to kill Dumbledore and I don't discern that as being any small feat.
Snape was willing to give Harry information via the Pensieve that should have directly led to Harry's death. That Harry survived Avada Kedavra twice is an anomaly, not a reflection on Snape's wilingness to allow Harry to die. Snape delivered the partial prophecy to Voldemort in 1980, anxious to curry favor. Even after he knew Voldemort was targeting the Potters, Snape was only interested in saving Lily -- does failure to prevent a premeditated murder imply culpability? In some jurisdictions, it most certainly does. (See the felony murder info.). I would say that, yes, Snape had likely murdered before his killing of Dumbledore. But I do believe that Snape's emotional quotient regarding the value of human life changed over the years (not that he was ever warm and fuzzy). Compare the following two passages for what I mean. 1981 versus 1997-98:
‘If she means so much to you,’ said Dumbledore, ‘surely Lord Voldemort will spare her? Could you not ask for mercy for the mother, in exchange for the son?’
‘I have – I have asked him –’
‘You disgust me,’ said Dumbledore, and Harry had never heard so much contempt in his voice. Snape seemed to shrink a little. ‘You do not care, then, about the deaths of her husband and child? They can die, as long as you have what you want?’
Deathly Hallows - pages 543-544 - Bloomsbury - chapter 33, The Prince's Tale
‘You have kept him alive so that he can die at the right moment?’
‘Don’t be shocked, Severus. How many men and women have you watched die?’
‘Lately, only those whom I could not save,’ said Snape. He stood up. ‘You have used me.’
Deathly Hallows - pages 551 - Bloomsbury - chapter 33, The Prince's Tale