Why don't Aliens murder humans?

I've never in my lifetime heard a story of ETs killing a human for any purpose..Why is this I ask? Do they have feelings for humans? Or are they here to analyze us and watch us evolve like an antfarm?
If they've been visiting for centuries there is only so much they can learn about one species. I ask you my friends do you know of a story/theory/whatever, that involves ET's murdering humans? An abduction with the person never returning...is not an answer..

Thanks for the input.
And if this has been discussed before I apologize.

what no one can answer this? one answer would be nice.

I only want to say this either we humans are a missing link for these ET. Meaning they need us for their own advantage, (i.e. their existence, labor.) Or we have a mutual agreement in some type of manner (i.e. they were/are some type of humans, or technology for studying)...or lastly they are interested in us for just the damn interest....I just cannot go into so many theories and explain them with detail.