I would say not, but that their taking of hosts and the civilization that was built on that encourages them to be "evil" by our modern, western, standards. And it is a fairly easy to trace this development. Initially the creatures they could take on was hosts were animals of limited physical and mental capacity and the Unas, which while having things like opposable thumbs, were probably little more culturally advanced than modern chimpanzees. Even now, after thousands of years of abandonment by the Goa'uld, they still were a pre-bronze age culture.
As the Goa'uld began exploring the galaxy they discovered left behind Ancient tech, learned how to interface with it, and in some cases replicate it. The Unas probably didn't have the ATA not being a race descendant of the Alterans, so that limits their access to many of the more advanced pieces of tech. Chances are the majority of those were destroyed as the Goa'uld tried to take them apart to learn their secrets. But they were able to figure out how to replicate their Hyperdrive and Healing tech (Sarcophagus). But since they were limited to Unas hosts, and possibly other species of non-Alteran descent by then, the Alteran healing tech wouldn't work well for them.
Eventually the Goa'uld, Ra specifically, discovered Earth and the Humans. As with the Unas, humans were at a very early stage in their development, bronze age or just before. Not only that the Alteran tech worked much better on the fast breeding humans than the Unas, a probable reason why Unas were abandoned as hosts for the Goa'uld (and fits in with the movie reason of humans being easier to repair) given Unas superior strength and durability.
So the Goa'uld, a space faring race that lives though the domination of a primitive species, comes across another primitive species. One without much of a language or a cohesive culture beyond extended family groups. Sure they have some basic tool use but so do modern chimpanzees and modern humans hunt them for sport or food. I doubt that would sway the Goa'uld, a species that has for generations, at this point, treated creatures of such development as beasts of burden. Not only that, with the genetic memory that Goa'uld are born with every new generation has that same impression of humans.
At some point though, probably around the time of the Earth Uprising, a group of the Goa'uld saw through the generations of stereotyping the humans as a beasts of burden. Having experienced the humans coordinated repulsion of Ra from Earth Egeria was one of the first to recognize the immorality of subjugating another intelligent species. That realization probably led to a re-evaluation of the cultural assumptions of the Goa'uld and eventually the Tok'ra.
Even so the Tok'ra aren't pro humanity so much as they are in opposition to Ra's status quo. You can see parallels to this in the US's early emancipation movements. Then the opponents of slavery had moral objections to the practice of slavery, but often were not welcoming of Africans into their communities.
no, they are NOT naturally "evil"
. Though it would still be interesting to see what supporting references people may be able to come up with.