The WTS does not believe they are pacifists because they support wars, Israelite wars with their enemies and Jesus Christ's wars against his enemies.
http://www.jwfacts.com/watchtower/war.php
Pacifism is "opposition to war or violence as a means of resolving disputes" (American Heritage Dictionary). Quite significantly, the Watchtower Society is not a peace church. Despite being neutral in war, Jehovah's Witnesses are not pacifists. A Witness may draw arms in self-defence. Jehovah's Witnesses are only forced to stay neutral until they are in the situation where they are required to defend themselves.
"The situation may be such that the only thing a person can do is to use whatever is at hand to protect himself or others. As a result, the attacker may receive a fatal blow. From the Scriptural standpoint, the one acting in self-defense would not thereby incur bloodguilt." Awake 1975 Sep 8 p.28 Should You Defend Yourself?
"True Christians love peace. They stay completely neutral in the world's military, political, and ethnic conflicts. But, strictly speaking, they are not pacifists. Why? Because they welcome God's war that will finally enforce his will on earth-a war that will settle the great issue of universal sovereignty and rid the earth of all enemies of peace once and for all." Awake! 1997 May 8 p.23
"They are not against war between the nations, and they do not interfere with the war efforts of the nations nor with anyone who can conscientiously join in such efforts. They fight only when God commands them to do so, because then it is theocratic warfare.
Were Jehovah’s witnesses today to claim to be pacifists, it would mean for them to denounce all the pre-Christian witnesses of Jehovah who took up arms to uphold Jehovah’s universal sovereignty and his theocratic nation of Israel. But this denunciation we cannot make. Jesus Christ never did so, and he is Jehovah’s greatest witness, who has earned the title “The faithful and true witness”. (Rev. 3:14) Jehovah himself is no pacifist. Neither are his witnesses such, although they are conscientious objectors." Watchtower 1951 Feb 1 p.70 Why Jehovah’s Witnesses Are Not Pacifists
Furthermore, pacifist churches such as Quakers are against the death penalty, which the Watchtower Society is not. The Watchtower Society "recognize the right of governments to do as they wish" in regards to the death penalty, noting that the Mosaic Law advocated the death penalty. (g 96 3/8 p. 23).