I would agree that Salazar Slytherin's antics with the Basilisk place him firmly in the territory of Dark Wizard. When given a choice between ousting Muggleborns and killing them, Slytherin chose to kill them via the Basilisk.
I think that what constitutes a Dark Wizard is complex and not easily explained. But I do think, if I were to summarize it in one sentence, I would have to defer to Albus Dumbledore who says to Harry in Chamber of Secrets, "It is our choices, Harry, that show what we truly are, far more than our abilities." (CoS - Page 333 - US Hardcover)
I may have some off-the-wall views of canon, but there are some wizards I see as dark, who are not presented as Death Eaters. For example, Gilderoy Lockhart took credit for others' heroic accomplishments and performed memory charms on the people he stole his stories from. That's pretty abhorrent, in my view. Barty Crouch Sr. helped his Death Eater son escape from Azkaban, yet put Barty Crouch Jr into another kind of prison: perpetually hidden underneath an invisibility cloak under the Imperius Curse. Barty Crouch Sr. was completely unethical and put his own needs before those of his community, knowing all the while that his son was dangerous and a Death Eater. He left his wife to die in Azkaban and to be buried there without any remembrance or ceremony.
There are the obvious baddies who seek power, control, superiority, and the desire to crush others they see as less than: Voldemort; Grindelwald; Dolores Umbridge; Lucius Malfoy (who one might argue had a bit of redemption at the end, when his concern for Draco trumped his desire to serve as Voldemort's right-hand man); the Death Eaters. These individuals all show sadistic, cruel, and racist attitudes.
But what about Dumbledore, who at one point in his life craved power so keenly that he partnered up with the very dark Grindelwald and dabbled in anti-Muggle rhetoric and sought to see the Wizarding community control the Muggle community "for the greater good"? Yet, Dumbledore's views change; his priorities shift when he is affected so deeply by the death of his sister Ariana. Some might argue that Dumbledore had his dark side.
I think one has to look at the motivation behind a wizard's actions before labeling them dark or evil. Yes, Harry uses two of the Unforgivable Curses, Cruciatus and Imperius, but the circumstances surrounding his employment of those curses (following Sirius's and Dumbledore's deaths respectively; searching for the Hufflepuff cup in Gringotts) differs drastically from the motivations of the Death Eaters who tortured Muggles at the Quidditch World Cup, murdered Muggle families for no apparent reason other than they were Muggles, and supported the subjugation of the majority of their community.
As Bellatrix Lestrange says in Order of the Phoenix, "You have to really mean it," in regard to wanting to hurt, kill, or devalue other human beings. I think that is wherein the line lies. It's not so much about throwing the occasional Unforgivable Curse (and I note here that Harry was unable to successfully cast Cruciatus on either Bellatrix or Snape), than it is embracing and living a deeply flawed, self-serving, and cruel life.
Yet I don't think we can ignore what goes into a person's character, to make them attracted to the dark side. For example, Snape grew up as a half-blood, neglected, in a domestically violent, alcoholic home -- of course his soul would be shadowed by that. And apparently he immersed himself in the Dark Arts as a means of escape and self-preservation (let's not forget that Snape was bullied at Hogwarts by James Potter and Sirius Black). J.K. Rowling has said a motivating factor for Snape becoming a Death Eater was his desire to belong, to be part of a group that accepted him.
And there's Bellatrix, Narcissa, Andromeda, Sirius, and Regulus Black, who were all raised in prejudiced families and were undoubtedly indoctrinated with racism and superiority all their lives; their families supported Voldemort's agenda. Although Andromeda Black and Sirius Black did not share their parents' beliefs, it is not unusual for children to grow up sharing their parents'/family's views by default, for they are not exposed to alternative beliefs and ideals. Once something like racism becomes ingrained, it is incredibly difficult to undo those beliefs; some embrace sadism, prejudice, and superiority because it is inherent to their personality (think Bellatrix -- she's way too bat* * * * to have not had the predisposition toward violence, cruelty, and control). Such wizards, I would think, would lean toward the dark side. And like attracts like, so they find each other, band together, and suddenly there's a dark force to be reckoned with.
So, yes, I think there are several things that influence a wizard to become dark or to follow the path of light: personality; belief systems; family background; the predisposition toward violence, prejudice, subjugation, and entitlement; good intentions gone astray; whether a witch or wizard is a follower rather than a leader; individual psychology; natural magical abilities; and the tendency to choose what is easy rather than what is right (as Dumbledore also said).
I don't think that being a Dark Wizard boils down to just the Unforgivables. It's far more complex than that.
ETA: I wanted to say that this is a great question!
