I understand the Goa'uld are naturally parasitic but are they naturally as evil and selfish as they are in the current age?
The Tok'ra show that some Goa'uld can be at least somewhat considerate creatures. Then the episode "Need" demonstrates that the sarcophagus has very negative mental effects on humans, including indifference for others, and paranoia.
Are Goa'uld just a victim of their own stolen technology, or are they inherently evil?
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.
The Goa'uld likely did not start out evil, but the detrimental effects of the Sarcophagus eventually led them down that path, roughly as stated in @Tyson's answer.
However, the Goa'uld have voluntary genetic memory - the Queens are able to pick and choose what memories and knowledge to pass to their offspring. Because most Queens are evil/ego-maniacal, that's the knowledge they pass on to infant Goa'uld - making the new symbiotes just as evil/ego-maniacal as their ancestors.
So yes, it is at some level the natural state of the Goa'uld to be evil as seen in the series. They are born with that state of mind, and as shown with Tanith, even as infants they will lie and deceive so that they can get their way.
It is unknown how or why Egeria's views of the Goa'uld were changed, and she is the only known symbiote to ever have such a change of heart.
The Tok'ra were created by Egeria through altering the genetic memories passed on, so they never had the Goa'uld mentality in the first place (which is one of the reasons they get angry if they're ever accidentally referred to as Goa'uld - they weren't born evil/ego-maniacal).
Okay I'm going to throw in my two cents.
I think the other posts more or less have gotten it down on point. That is more or less due to the genetic memory characteristic of the Goa'uld and their prolific use of the sarcophagus (which we do know can cause, megalomania, a sense of superiority and various other effects on the mind) cause the Goa'uld to be born predisposed to being evil.
But now while this is speculation, I believe that if not exposed to a sarcophagus a Goa'uld has the potential to more or less become less evil (due to a change of philosophy or viewpoint). I reason for thinking this is because each individual Goa'uld even with the genetic memory still develop their own personality, also couple that with the fact that the genetic memory may not be perfect (you have to understand the Goa'uld are still living creature so and are going to be subject to mutation meaning that their genetic memory may go out of whack sometimes in rare cases).
So does my belief that Egeria most likely just for whatever reason abstained from using a sarcophagus for a significant period of time that or is simply was a mutant Goa'uld had a lesser predisposition to being evil.
So yeah Goa'uld are basically born evil but with proper environment factors may have a change in philosophy (remember not all of the Tok'ra were all from Egeria's broods some in fact were Goa'uld who had a change of heart although this was very rare).
The best evidence for the Goa'uld being innately evil is Tanith, who has a larval Goa'uld was able to lie to his host, Shau'nac during kelnorim in order to infiltrate the Tok'ra. Similarly, Teal'c's interactions with his symbiote ("Junior", as O'Neill called it) involved the symbiote showing his father's death in order to intimidate him. This suggests that even before they are mature, and have had frequent use of the sarcophagus, the Goa'uld are innately full of malice, which could be considered "evil".
The Teal'c created in the altered timeline of Season 8's Moebius two-part finale that ended up to ancient Egypt communicates with his symbiote via kelnorim. This is part of the conversation they have:
Is this not what you wish? it asked. What all things wish, to serve their gods?
I am Jaffa, Teal’c thought, carefully. It is not my place to be a host.
You fear it, the symbiote said. Why?
[…Teal'c has some recollections of a Shau’nac in his timeline becoming a host to a symbiote…]
And I do not wish it to be so! I will not leave you under these conditions, I will not take a host as that one did —
As do all your kind, Teal’c thought.
No! I will not! The spark flared, its anger crackling through both of them. I will show you — find me a willing host, or none at all.
You will stay with me, then, until you have a host willing to accept you? Teal’c asked. Though it may be difficult, and painful for us both?
I will, the spark answered.
So this symbiote is a larval Goa'uld that, during its first conversation with its incubating Jaffa, shows an uncharacteristic respect for the rights of others, much more like the Tok'ra, and we find out later that it sincerely believes this. You'll recall that this is a Teal'c from an altered 2005, so his symbiote has several thousand years of genetic memories, yet that doesn't seem to be influencing its views. In fact, the novel reveals that
This symbiote is Egeria, the progenitor of the Tok'ra, meaning that while modern Tok'ra may have been "taught" to be good by Egeria, Egeria herself did not have any such influence.
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.