Why is the baby water buffalo scene more disturbing than the death of one of O’Brien’s platoon mates, Curt Lemon?

In "The Things They Carried", the baby water buffalo undergoes a significantly more traumatic experience than Curt Lemon. One of the unspoken points of the text is that Curt Lemon signed up to potentially lay down his life, and the baby water buffalo had no part in it. Additionally, the baby water buffalo is a young animal, completely innocent of the conflict.
Beyond that, however, the baby water buffalo suffers terribly in its experience while Curt Lemon dies immediately. The animal is shot repeatedly in non-life-threatening ways, which just cause pain and anguish until it sinks down and waits for death. Curt Lemon, however, dies a swift, painless death that ends his suffering without even realizing what happened. The baby water buffalo has to feel every bullet and ends up sinking down in quiet despair. This makes the scene with the baby water buffalo significantly more disturbing than Lemon's demise.

I agree that the baby water buffalo's defenselessness certainly plays a role in how disturbing its death is. In addition, the fact that it is an animal that cannot possibly understand what is happening to it is only augmented by the description of it as a "baby" animal. Such a word carries heavy emotional connotations: we think of babies as innocent and beautiful, something to protect from harm—not something to which one does unspeakable harm. The fact that Rat is so vicious in his attack against this young and innocent creature adds to our impression of the scene.
Moreover, this creature is not to blame for Curt Lemon's death or his friend's rage. We might understand more if Rat raged against something or someone who could be held accountable or blamed, but taking it out on a defenseless baby animal who had no part in Curt's death seems especially unwarranted and cruel.

The baby buffalo scene is likely more disturbing because the animal is defenseless against the onslaught of Rat Kiley's bullets.
The text tells us that, unlike Curt Lemon, the baby buffalo does not die immediately. While Lemon dies an instantaneous death after stepping on a booby-trapped mortar round, the baby buffalo dies in stages.
First, Rat Kiley shoots the baby buffalo through its front right knee. Then, he shoots off an ear and pumps bullets into the animal's hindquarters, hump, and flanks. Still in a state of anguish over the loss of his friend Curt Lemon, Rat continues by shooting off the baby buffalo's mouth, nose, and tail. He also shoots the animal in the ribs, stomach, butt, and throat. 
The text tells us the baby buffalo does not make a sound throughout its torturous ordeal. Instead, it sinks down into the ground and lies still, patiently waiting for its deliverance from pain. By this time, the only thing capable of moving is the animal's enormous black eyes. 
So, the baby buffalo scene is infinitely more disturbing because the defenseless animal suffers an excruciating death, one it does not deserve. Although one can argue that Curt Lemon also did not deserve to die, his death was instantaneous.