I can see how many believe that Aikido is pacifistic, however, I think it is all in your perspective. I agree that if you only consider that we focus on no injury and non-violence to the attacker, then I would agree. Even when reading OSensei's teaching, at first glance many might come up with the same conclusion.
I think if you really and earnestly contemplate the teaching of OSensei, you will uncover the truth and will realize that the art he has passed down to us all is the most violent and active of all the martial arts. Becoming one with our attacker actually places us in the ultimate and most deadly position of all. We are focused on ridding the world of hatred and violence, and when we enter into the attack, our counter is swift and cuts like the sword deep into the hearts of our attackers. Is our aim to hurt or kill the actual attacker? No, not at all. Is our intent to kill the evil with which the attack was committed? Absolutely. For example, in many arts once a take down is complete the defender usually continues with some blow that either knocks out the opponent or may even kill them. Negative energy will breed more negative energy. Through the techniques of restraints and holds (positive energy) we are striking at the evil within the attack and not the attacker.
I've went on too long. I would say we're not pacifists, but extreme activists. When we come to the realization that we enter into the attack to purge/kill the evil intent and not the opponent, we will have accomplished what I believe OSensei was imparting to us all.