Actually, the Hork-Bajir used to be pacifists, they aren't anymore. Before the Yeerks came, the Hork-Bajir were "too stupid to know how to kill" as Dak Hammee put it. Now that they have learned how to kill, and have a heavy motivation to do so, they don't seem to mind doing it. After all, the free Hork-Bajir often attacked Yeerk camps to free more of their kind, and they also fought to protect their home from invasion.
Sad as it is to say it, the Hork-Bajir aren't pacifists anymore...