The Hork-Bajir don't seem to care that they're being mutiliated since they have the motto "Free or Dead!" and would view the scars proudly(!) but humans are more complex than that. Besides, I think any injury is not as bad as an EYE INJURY. Tobias could have just tore the guy a few scars on his skin, but NOOOO, he rips out his whole EYE.
I always found this disturbing about the Animorphs, that at times they can rationalize doing almost anything because the Controllers are "no longer human." Hey, ****s, their MINDS are still aware! (shakes head)