I'd have to go with a solid No on this one.
I think that the definitions of "supreme act of evil" will vary from person to person. We can find minute examples of this in the way wizards treat house elves or even better examples of how they see Muggles.
House Elf Example
Hermione deems it despicable to order a house elf to do anything and more so if they aren't even being paid. The Malfoys treat Dobby much much worse than just giving him orders or not paying him a wage, yet see no problem with this.
This is basically the underlying point of the entire series. Blood bias seems to have really started with Salazar, it then grew in popularity again after Wizards separated from the Muggles with the Statute of Secrecy. The pure-blood advocates naturally see nothing evil in their selective nature nor do they regard the Muggles as equals to Wizards.
Now from this answer.
We know that Voldemort did in fact use Muggles to create his Horcruxes, interestingly enough he seems to have killed 3 Muggles and 3 Wizards emulating his own Half-Blood status also interesting Is that the 3 Wizards were female and it is entirely possible the 3 Muggles were male although unknown due to the Tramp and Peasant the sexes of his kills would serve to further emulate his parentage. I digress. The main reason for pointing this out is that Voldemort, although he despised Muggles still deemed them a worthy kill because of his Father.
Down to the crux of the matter
The reason why all this is relevant is because evil is extremely subjective and based largely on a personal moral compass. Note @Richards comment about Hagrid, he is not technically a person but most people interacting with him would deem him as such. I think that depending on person what can be murdered in order to split your soul changes.
Ambiguity with the terms
what would happen to the wizard so determined to evade death that he would be prepared to murder many times, rip his soul repeatedly
Half-Blood Prince - Chapter 23 - Horcruxes
Here we have Dumbledore talking about multiple murders to split a soul.
“Listeners, I’d like to invite you now to join us in a minute’s silence in memory of Ted Tonks, Dirk Cresswell, Bathilda Bagshot, Gornuk, and the unnamed, but no less regretted, Muggles murdered by the Death Eaters.”
Deathly Hallows - Chapter 22 - The Deathly Hallows
Here we have a news report which mentions one non-human, Gornuk, as being murdered by Death Eaters.
Like with humans in real life we all have a difference sense of what constitutes murder, from Vegans to Hunters for fun and from Pacifists to Serial Killers. There is a huge variation in moral compasses which allows for the ambiguity with the term 'murder'.
universe Potterverse where we have multiple beings of intelligence with the following capability:
Grogan Stump, the newly
appointed Minister for Magic, decreed that a “being” was “any
creature that has sufficient intelligence to understand the laws of
the magical community and to bear part of the responsibility in
shaping those laws.”2
Troll representatives were questioned in the
absence of goblins and judged not to understand anything that
was being said to them; they were therefore classified as “beasts”
despite their two-legged gait; merpeople were invited through
translators to become “beings” for the first time; fairies, pixies,
and gnomes, despite their humanoid appearance, were placed
firmly in the “beast” category.
Fantastic Beasts - What is a beast?
The definition of Murder becomes more ambiguous.
Even more ambiguity is thrown into the mix when you have the intermingling of these species with 'humans' like with Hagrid.
In common usage, the word "human" generally refers to the only extant species of the genus Homo — anatomically and behaviorally modern Homo sapiens. Its usage often designates differences between that species as a whole and any other group or entity.
In our world we our the only extant species, but we do not know enough about the beings in HP to call them Human or not.
So there you have it, killing a being in my opinion would constitute murder IF the murderer deemed it as such and the murder is premeditated
I have not covered intent in this answer other than for the summary, I feel this answer does intent much more justice than I could without making this already long answer longer.