This question has come up before, so you will likely be interested in this answer, which references the book "Men Against Fire" which that particular episode took its name from.

To add on to u/Georgy_K_Zhukov's answer a bit, a translation of "Battle Studies" is available online now and Du Picq himself seems to have drawn very different conclusions from his experience than Grossman or Marshall did. Rather than troops simply not wanting to kill, Du Picq focused on the impact of smoke, noise, stress, and the instinct of self preservation. In fact he believed that the phenomenon of troops firing into the air or without aiming had more to do with nervous troops being too eager to kill the enemy in order to save themselves: "He is instinctively in haste to fire his shot, which may stop the departure of the bullet destined for him. . . Even the bravest and most reliable soldiers then fire madly."

In situations where soldiers were reluctant to open fire or engage the enemy he instead argues that the soldiers are afraid of exposing themselves to danger and retaliation in the process. Rather than an opposition to killing in general the anecdote Du Pict gives is "He [The average soldier] is not anxious to light the fuse that is to blow up the enemy, and himself at the same time."