Did the writers of Star Trek need to create numerous violent species to propel the show over the course of its 40 + years of existence? Yes. Did they create too many? That depends. Given the science behind intelligence, it might be said they didn't create enough.
Given the number of stars in our galaxy (100 to 400 billion are estimated ranges and the corresponding hundreds of billions of potential planets out there) there could be any number of life forms doing any number of things. The scientific thought says, predators (and omnivores) will likely develop intelligence since you don't need intelligence to sneak up on a plant.
Such active and predatory intelligence will give rise to both dominance and survival of the fittest social hierarchies and likely create species who will see conflict and competition as natural drives even after they become cooperative enough to develop space-flight.
From a production standpoint:
We have to consider both the needs of the market (the viewers - what they will watch and their reasons for watching it) and the needs of the production company (who need to get viewers in order to get advertising in order to pay for the continued existence of the show).
A show needs consistent elements - characters (protagonists and antagonists) the viewers can relate to. They need enough consistent ones that they can reuse costumes for and enough new ones to keep people guessing as to what the next week's threat is going to be.
Writers need to have plot elements they can rely on for the relatively simple stories presented in at least 1/3 of all Star Trek stories - crew meets unknown element, crew interacts with unknown element, unknown element becomes known, crew defeats or escapes unknown element.
All of this is to say, Star Trek was like any other business in some regards. It was forced to make concessions regarding its players, its enemies, its models, its cinematography and not all concessions were created equal.
As Star Trek became more famous, in later iterations it was possible to create many more new races whose motivations were less clear cut and allowed some room for actions beyond violence such as:
The wormhole aliens (The Prophets) who lived in the Wormhole near Bajor. We never know what they want, or why they do what they do, only that they have some affection for Bajor and its people.
More malevolent lifeforms such as Species 8472, whose primary motivation was the extermination of all lesser species (at least at first).
There was the expansionist role played by The Borg who simply wanted to absorb your species and technology. They didn't consider themselves good or bad.
Or some combination of the other motivations such as the Breen or the Dominion.
Violence, as depicted in Star Trek, would be expected from at least some degree of the aliens in the galaxy given the forces that would drive a species into space: the urge for competition, territoriality, the urges for expansion, the urge to dominate other life forms would all be urges, at least some species would take into the last great frontier. It would be a feat if a species could overcome its biological urges long enough to meet alien life without killing it out of a xenophobic fear of the Other.
Incidents in Human history should make this abundantly clear. Any time a low tech culture met a high tech one, the low tech culture tended to be consumed, enslaved or eliminated completely.
Why would first contact in space be any different? If anything, Rodenberry showed great restraint in his presentation of many aliens. Most were willing to put aside their cultural xenophobia once aggressive tactics failed, political machinations however continued for decades; see Romulans and Cardassians. The Klingons eventually gained some measure of respect for humanity and found reasons for effective camaraderie.
In Summary:
Ecosystems are potentially dangerous and inherently violent. Why would space and the expansion into a vaster ecosystem be any less dangerous than one confined to a single planet. If anything, entering the space ecosystem could mean a species exposes itself to complete annihilation the first time it meets a threat with superior technology... and the means to reach the weaker system's home worlds.