最后一句这样写首先是因为需要押韵
So on we worked, and waited for the light,
And went without the meat, and cursed the bread;
And Richard Cory, one calm summer night,
Went home and put a bullet through his head.
其次是shoot a bullet through his head, the expression is more artistic than simply "shoot himself dead".

觉得是写出了人生的变换莫测和深不可知，对于别人的艳羡可能覆盖了他本人对生活的感知，所以主人公的令人眼红的梦醒生活却成为他本人的梦靥，只能通过子弹来结束自己的生命，另一方面也暗含了人们梦幻生活的的破灭和不可求。