How valid are the claims Mother Teresa was a 'sadist' who made the poor suffer?

EDIT: This post, innocuous enough when I wrote it, has seen a surprising amount of vitriolic bad faith responses calling it into question from people who either have clearly not actually read it, or have deliberately chosen to deform my words. I am at this point getting exhausted of having to defend my post against such comments every half an hour or so, so I will at this point stop responding to them.

But let me make one point perfectly clear: at no point does Mother Teresa, or mainstream Christian doctrine, state or even imply that is preferable in any way to suffer than not to. Furthermore, there is no evidence of any kind that deliberate cruelty was visited on the people in her hospices with the goal of increasing anyone's suffering. The very worst Mother Teresa can be accused of in good faith is incompetence. The claim of sadism is one that cannot reasonably be made from any of Christopher Hitchens' alleged sources.

Now, there is always more to be said on the topic, and it seems your question goes far beyond the simple question of Mother Teresa's alleged "sadism", but that one topic can at least easily enough be answered. The claim, as you've said, largely comes from Christopher Hitchens' book The Missionary Position (charming title Mr. Hitchens), where he writes the following:

The point is not the honest relief of suffering but the promulgation of a cult based on death and suffering and subjection. Mother Teresa [...] once gave this game away in a filmed interview. She described a person who was in the last agonies of cancer and suffering unbearable pain. With a smile, Mother Teresa told the camera what she told this terminal patient: 'You are suffering like Christ on the cross. So Jesus must be kissing you.' 1

First thing I will note about this quote is that, since Mr. Hitchens fails at any point in his book to actually properly give his sources, I am unable to find the interview in question. However, considering his track record, it's quite likely that there's at least some manipulation of the context going on here, as he repeatedly does throughout his book. The book, after all, is far more character assassination hit piece than anything resembling a scholarly work or a piece of authentic journalism.

Since I can't look at his primary source source myself, I'll just have to take him on faith that the interview even exists, and will certainly be unable to call him out for misrepresenting her words in it.

Now, onto the actual quote. on its own, even removed from its context, it's hardly the slam dunk that you'd expect from Mother Teresa "giving the game away". She could after all just be expressing that Jesus would be sympathetic to this person's suffering. However, Hitchens has definite proof of sinister intent: a 1994 article by a visiting Dr. Robin Fox in the Lancet notes that painkillers are not being given to the patients! There it is, the undeniable smoking gun of Mother Teresa's attempt at causing suffering in her hospice!

(As an aside, since this time Hitchens quotes extensively from the article, it was relatively easy to locate. A cursory read shows that, when he quotes it, Hitchens is careful to quote around and cut as needed, such that he leaves the entirety of Fox's criticism, and omits the totality of his praise, which he dismisses as Fox's "slightly raised-eyebrow politeness").

However... That's not quite true. In his article, Dr. Fox notes that:

I saw a young man who had been admitted in poor shape with high fever, and the drugs prescribed had been tetracycline and paracetamol. 2

First, in his entire article, Fox at no point claims that strong analgesics that could be administered are being denied to the patients, as Hitchens extrapolates. What he does note is the lack of them, and that some (weaker) painkillers are being prescribed instead. But if Mother Teresa truly believed that it is preferable to suffer than not to, why would she give this much to the young man in question? Why would she bother giving him antibiotics? Would she not deny both in order to maximize his suffering, as Hitchens implies?

(The reason she might not provide stronger painkillers is not one I am personally very familiar with, but the post on /r/badhistory that you linked goes in great depth on the topic, and the sources seem both reliable and accurate to the poster's interpretation, so I would be inclined to believe them when they claim that she didn't because she couldn't due to Indian laws and regulations regarding opiates.)

What Mother teresa is actually expressing in her quote is the perfectly mainstream Christian concept of "Redemptive Suffering". In short, by suffering as Jesus himself suffered, and by offering up one's suffering to God for the sake of others, one can be in communion with him. As Paul wrote to the Colossians:

Now I rejoice in my sufferings for your sake, and in my flesh I do my share on behalf of His body, which is the church, in filling up what is lacking in Christ’s afflictions. (Col. 1:24)

The idea is that Christ suffered for our redemption, and our suffering can be offered up alongside his to help redeem mankind. This is clearly demonstrated by another quote from Mother Teresa that Hitchens fails to mention in his work:

Suffering will never be completely absent from our lives. So don’t be afraid of suffering. Your suffering is a great means of love, if you make use of it, especially if you offer it for peace in the world. Suffering in and of itself is useless, but suffering that is shared with the passion of Christ is a wonderful gift and a sign of love. 3

Note what is conspicuously absent from this quote: that it is better to suffer than not. What she is actually saying is that, dedicating our suffering for the redemption of Humanity can give it meaning, can make it into a gift of love to the world, and thus make it easier to bear: you are no longer bearing your suffering alone, you are bearing it alongside the Christ. Thus "You are suffering like Christ on the cross. So Jesus must be kissing you."

I would be willing to bet that there's more than just that one tiny snippet in the actual interview, but unfortunately Hitchens neglected to include it. If his treatment of Fox's article is any indication, it's likely because it would unmake his attempt at painting her as a sadist monster who reveled in the suffering of her wards. In short, however, she is not saying that we ought to suffer, but that when we suffer, we can give our suffering meaning through Christ. It's a silver lining on a dark cloud, and she still prescribed painkillers to those who needed them, within the limits of availability.

At worst, one can accuse Mother Teresa of being an incompetent hospice manager. At best, of doing the very best possible considering the incredibly adverse circumstances. But what is a completely unreasonable interpretation of the available information is that she was secretly a sadist running a "cult based on death and suffering and subjection".

And the easy counterargument is this: if she had been a fanatic obsessed with encouraging suffering and death, why start the hospice at all? It would have been better to leave them to die and suffer on the street, the better to commune with God. After all, according to Hitchens' own "damning" source:

A walk through that squalid part of the city will show you disease and degradation on a grand scale. The fact that people seldom die on the street is largely thanks to the work of Mother Theresa and her mission. 4

1: Hitchens, Christopher, 1995. The Missionary Position: Mother Teresa in theory and practice. London: Verso. Page 41.
2: Fox, Robin, 1994. Calcutta Perspective: Mother Theresa's care for the dying. The Lancet, 344, page 808.
3: Mother Teresa, 1977. No greater love. New World Library. Page 136.
4: Fox, Robin, 1994. Calcutta Perspective: Mother Theresa's care for the dying. The Lancet, 344, page 807.