Is it ever more loving to end your childs life then to let them suffer(disease)

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I saw an episode of Law and Order SVU once that really brought me to tears and I just couldn't figure out how I felt about it. A woman had a child who ended up being diagnosed with Tay Sachs I believe it was and her doctor gave her sleeping pills to crush up and feed her baby in her formula. I am a person who gets very emotional when I hear about someone hurting a child let alone there own, but this one got to me very differently. Even though I could never do it I could see (of course its a movie and made to be dramatic) that she truly did it for the child, so that she wouldn't suffer the very horrible, painful disease that would lead to her inevitable painful and extremely early death. But the real question is is what she did an understandable or forgivable thing, or was she completely wrong and had no right. I don't know that I could judge someone who would do this because I could never imagine knowing my child would go through such horror. I really hope this doesn't tern into a big angry discussion, just something that another post I read reminded me of.