What are some quotes and analyses about the motif of morality in Life of Pi?

Life of Pi can be read as the conflict between human morality and animal instinct under desperate circumstances. At its core, it is a story of survival and what toll that survival might have on human morality. 
Much is made of Pi's vegetarianism. For religious, cultural, and moral reasons, Pi does not eat meat, yet when faced with starvation, the need to survive triumphs over any qualms he might have about eating meat.

You may be astonished that in such a short period of time I could go from weeping over the muffled killing of a flying fish to gleefully bludgeoning to death a dorado. I could explain it by arguing that profiting from a pitiful flying fish's navigational mistake made me shy and sorrowful, while the excitement of actively capturing a great dorado made me sanguinary and self-assured. But in point of fact the explanation lies elsewhere. It is simple and brutal: a person can get used to anything, even killing.

This is a good quotation showing the evolution of Pi's morality. When he was first shipwrecked, the idea of killing even a fish drove him to tears, but very quickly, he learns he must kill to survive. Later, we have accounts of him relishing drinking turtle's blood and even eating human flesh. 
The physical needs of the human body, from feeding to defecation, are portrayed in an animalistic fashion. However, there is one instance where Pi is talking about water and the pure, unadulterated joy of slaking his thirst:

I tell you, to be drunk on alcohol is disgraceful, but to be drunk on water is noble and ecstatic.

Fulfilling the animal requirement of hydration in this instance is elevated to something glorious, almost religious in its experience. Pi notes that being drunk on alcohol is immoral but to be drunk on water, in the process of tending the body's needs, is a good, moral, and even noble thing. 
The brutality of survival under such circumstances is summed up in chapter 90 with this quote:

This was the terrible cost of Richard Parker. He gave me a life, my own, but at the expense of taking one. He ripped the flesh off the man's frame and cracked his bones. The smell of blood filled my nose. Something in me died then that has never come back to life.

Pi survives but at the expense of human life. This quote must be viewed alongside Pi's second story, where it is Pi himself, not Richard Parker, who commits murder. Murder is held in most cultures as the ultimate transgression. There is a part of Pi that will never recover from this, even though it happened due to a direct threat on his life and under unimaginable circumstances. Here, Richard Parker can be seen as the ultimate triumph of animal survivor instinct over human morality.