First, there is a way to tell it to not do that. With Google Now, you simply tap the menu and say "No more notification like this". With the assistant, you will probably be able to ask directly.
Second, let's be honest, humans fail pretty often too, so that's just a weak argument.
Lastly, I think it's unfair to dismiss a new technology just because it could maybe fail, without having even tried it.
The awful success cases are far more interesting than the awful failure cases.
Your noose example is pretty contrived, however.
How about sleeping pills? Opiates? Local extortionist cult?
There's not only the "morally reprehensible" metric ("Don't be evil"); there's also the "absolute PR catastrophe" metric that printing such an ad for a rope would mean.
I think the real issue is the casual deception which you just fell for: It isn't "your" electronic secretary, and the thing it just did might actually be a "good job" from the perspective of those who control it.