Hens and cocks establish a strong pecking order such that there is a top male and a top female. (The alphas). Below these each individual learns his / her place. When new chicks begin to become independent of their guardian mother there is often a bit of fighting and occasionally injury. This is especially noticeable with the young cocks. When two individuals feel strongly equal the fighting can be very bad and serious injury can occur. Fighting to the death is very rare (unless man interferes as in cock fighting).
Normally chicks soon learn their place and a few pecks from the adults sort them out.
Last night it went wrong for one of Snowflake's 2 remaining chicks. A single peck to the top of the head and life was extinguished.