I often here the counter argument about survival instincts of other species causing them to kill without cause, which is completely incorrect. I'll counter the three main examples and lay this matter to rest.
First they often cite chimp or gorilla behavior, the often violent reactions to perceived invaders. There, I just explained why they kill each other without even touching on the science and why this example is not valid. But I'll continue anyway.
Every species has evolved a certain set of traits that help it's survival, not as individuals but as an entire species. Territoriality is one of those very effective traits to help a species, even humans have this trait but I'll get into that at the end. It is a method of ensuring there are enough resources for the group, often maintained by closer relatives and any others who threaten the resources are seen as enemies.
Us failing to see a valid reason for these behaviors is not proof that there is no valid reason. We anthropomorphize far too often, and in doing so we attempt to apply human logic to actions which are often instinctive.
A better example would be how the gorillas treat kittens when there is plenty of resources and security, many videos and stories of this type permeate the internet now. They are kind and gentle, even forgiving to a fault. Remove the instincts and they behave much more humane than most humans.
The second example are male lions killing cubs, another instinctive behavior. Again, that alone demonstrates that it is necessary, they do not have the social structure that allows the luxury of any other behavior, unlike humans. Male lions also do not kill them all, their instinct even has limits.
This behavior allows them to strengthen the pride and keep it stronger. It is a trait resulting in thousands of years of natural selection, because it works to help with their social structure. When placed in a situation which lacks the pressures of the wild, the instinct rarely kicks in, their environment is different and favors different traits.
So yes, both of those examples are not relatable to human logic and human caused suffering. They are both instinctive and cause by environment, by altering the environment the instinct is not triggered and they do not harm others without reasons easier for humans to comprehend. So stop anthropomorphizing before coming up with some counter argument.
The third, and I saved the best for last, is the domestic felines' tendency to play with their prey. Another example of anthropomorphizing, and one that is regularly misused. The "toying" is actually caused by a suppressed instinct to hunt, we selectively bred them like this so it's our own fault anyway.
When in the wild, felines have to battle for food, they have to chase and strategize to capture their meals. We selected the most playful felines for hundreds of years, playful being how we viewed this instinct, and bred them to a point in which this instinct is no longer bound to the need for food.
Yes, we made domestic felines sadistic to suit our own desires. We altered an instinct that was refined by natural selection without considering the long term consequences of this genetic engineering. There are many more examples of this problem in other domestic species and even ourselves.
So thus is not an example of animals killing needlessly, it is an example of why genetic engineering needs to be more scientific than it once was.
As humans we have one rare trait, seen in only a few other animals, it is the ability to alter our own instincts through knowledge, understanding, and willpower. We have no excuse to harm any other animal without a good reason, none. This us very important because of our population growth, too many humans causing unnecessary harm will destroy the ecosystems and make the planet uninhabitable for our own species.
We needs these other animals to survive, plain and simple. Nature needs the diversity to ensure the survival of these other animals. Our interference in any way will always cause more harm than good.
We have engineered some species for our food supply, there is not need to hunt the wild species now. So yes, humans are the only animal that causes unnecessary harm.