Data wasn't being cruel, merely honest.
There usually isn't a difference. Although in this case, Data has no basis for being honest, as he doesn't possess the necessary facts.
There's a big difference. You can be honest without being cruel (it's called tact.) Data wasn't cruel here. Given the magnitude of Marr's decision, Data's response was carefully modulated. And he
did possess the facts, insomuch as anyone could know the mind of a long-dead boy, via his detailed logs.
Cruelty also implies intent to hurt. But Data is portrayed as innocently trying to help Marr understand how her son would have felt:
Data, incapable of fathoming her psychotic break, can
only respond on a literal level.
DATA
I do not find such a file in your
son's journals. However... from
what I know of him by his writing
and his memories, I do not believe
he would be happy. He was proud
of your career as a scientist,
and now you have destroyed it.
You say you did it for him, but
he would not want that. Yes.
He would be very sad now.
(innocently hopeful)
Does that help, Doctor Marr?
Note how the script explicitly states that Data was innocently hopeful/helpful here -- and that's exact how Spiner played him, without a trace of cruelty. Note also that he begins his assessment by saying "I believe," acknowledging the rest to be his opinion based on the facts of the log entries. And that "I believe" clearly applies to the "He would be very sad now" statement as well.
Of course we can decide we have the ethical right to kill animals for food. But it then follows that we cannot logically have the ethical right to deny other lifeforms the killing of humans for food.
Really? Because there's no logical difference between killing human life and any other life? The Rwandan Genocide is ethically equivalent to crop-dusting a corn field? (I was going to say the Holocaust but I wanted to avoid Godwin's Law

) Obviously you have to draw the line somewhere, otherwise murder is ethical. Oh, I forgot:
Well, there we have it. Ethical is whatever the strongest person says it is.
The best episodes of Star Trek, like this one, revolve around finding ethical high ground in difficult situations. Star Trek is about ideas and ideals. Might makes right is about as far from Trek philosophy as you can get.
Timo, I get the feeling you don't really believe some of the positions you take, you just enjoy the mental gymnastics in trying to justify them. I'm not calling you a troll per se, just a very frustrating person to discuss Star Trek with :P