There are actually a couple of questions here that could use discussion. First is the nature of violence in itself. Second is the question of whether martial arts are inherently violent.
Let's address the first one. The narrow definition of violence is usually physical. It involves the harm or destruction of a human or so-called "higher" animals. Most of us don't think about the bugs we kill or the slugs we exterminate in our gardens… But every culture has developed at least one if not several models for what constitutes "violence". In India you can find people who consider it violent to do any harm to any sentient creature. They wear masks in order to avoid inhaling an insect. In our own culture you can find a number of folks who believe that doing any harm to animals is violent, even if it potentially saves human lives.
But beyond just the physical violence against sentient beings you can expand the definition to include destructive acts against even material objects. The term "vandalism" refers to the damage or destruction of material objects. We tend to view the seriousness of such an act by the amount of human investment in the object which was damaged. A work of art being vandalized is usually seen to be somehow worse than vandalizing an auto for instance.
All of the above examples have one thing in common, though. All of them involve "Intention". A tornado or hurricane is incredibly destructive. The energy is certainly described as violent since the result is sometimes complete destruction of whatever they encounter. So clearly "violence" exists in nature, even when a being with intention is lacking.
Also, how about TV? Is a show which depicts violence really "violent"? Certainly no one was really killed or injured. The depiction was fictional… Is that really violent?
It is quite clear that human beings are programmed to be capable of violence as a survival mechanism. It is also clear that this programming, in the modern world, actually constitutes one of the major threats to the survival of our species. Violent behavior that once was required for our survival has become dysfunctional in the modern world.
Aikido represents a practice, a form of training for human beings to be able to take the violence which is present in their world, and even within themselves, and learn to control it, channel it, and potentially even convert it to something else. Budo should be about perfecting oneself. Yes, the techniques we practice are potentially violent. But is the practice violent? Especially in Aikido we are constantly practicing techniques that were designed specifically not to be destructive. On the other hand they could be quite destructive if that were necessary.
There are still instances in which it is functional to be able to be violent. To protect oneself or another from harm is clearly a natural survival function. But most violence is the product of fear, not clear intention. Martial arts training should be, first and foremost, about dealing with that fear. You simply can't do good Aikido if you are fearful. In fact you can't do any martial art from a fearful state. So training takes one to a place in which fear does not control your actions.
I have met many high level martial artists. Many of these are combat veterans, and are quite capable of executing the most violent actions possible. Yet they are the least likely of anyone I know to do so. If you met these folks you would have no idea that they were capable of such actions. They are universally quiet, humble, and thoughtful. They would be the first to walk away from any potential conflict unless it became unavoidable. Then their actions would be quick, decisive, and only what would be needed to end the threat.
Martial arts training is also about taking responsibility for ones own actions. It is about being present and aware. It is about developing clear intention. All of these traits are specifically those that make other forms of "violence" less likely. As a culture we destroy our environment because we are unwilling to take responsibility for the consequences of our actions. We allow whole segments of our population to exist in poverty because we don't wish to pay attention to what is right in front of us. I think O-sensei's vision for Aikido as a way to make the world a better place comes directly from the fact that training should make us less fearful, more sensitive, more willing to take risks, more humble, and more aware and more responsible.
So you could say that martial arts are violent in that we practice techniques that can be destructive. But I would actually maintain that martial arts, properly taught, are in fact not violent at all. They lead to a reduced likelihood of violence, less internal conflict, less external conflict. Most martial artists I have met have an acute awareness of the fragility of human life and are very respectful of all life for that reason.
So the problem is not violence in itself. If we didn't have the violence of our immune system going on moment by moment we would be dead. If we didn't have the ability to defend ourselves from those members of our own species who are intentionally violent we would be victims. Violence (or destruction), in the sense that it is the flip side of Creation, is simply a fact, neither inherently good or bad. What produces unnecessary violence is fear. Fear causes most forms of human violence including physical and emotional violence. Good martial arts training reduces fear.
So I would say that. Despite the fact that we use techniques which could be violent in their result, our practice is precisely the opposite of violent. It fosters awareness, personal responsibility etc. All of which make violence of any kind less likely. And on those occasions in which violent action is necessary to protect the common good, we are able to do that as well and do it with the minimum creation of negative Karma to go forward.