What has been the most cold-blooded thing you've heard a person do to another?

It's hard to pick ONE ... I mean, given all the genocides committed in the 20th century ...

Here's a few

Some men raping babies and children. Some men marrying little girls and impregnating these children claiming it their religious right.

There's so many.

According to Viking lore, they performed the Blood Eagle, a ritualized method of execution. The victims back was opened with a knife, his ribs were cut from his backbone and the lungs were pulled up and onto his shoulders, while he was still alive.
This was portrayed in the TV series Vikings. Pretty disgusting.

Numbers 31:17

Now therefore, kill every male among the little ones, and kill every woman who has known a man by sleeping with him.

This means they killed pregnant women and children. Where's the anti abortion movement?

i have been to prison so put that in your imagination. I have seen dead bodies while working at a sheriff dept vehicle maintinence.  Not pretty what bullets do to a human body

If the Bible is quoted out of context as an example of something that is misleading, I consider that quote to be pretty cold-blooded against those who are perhaps not as sophisticated with regard to such insinuations as they might be.

It is we humans that are capable of the greatest atrocities against our fellow humans---and the mental harm that we may do them tends to be worse than any physical pain we can inflict upon them.

The word integrity comes to mind, but of course I could be oveerreacting.

Too many times, a parent makes the decision to have a baby after finding out that they're pregnant; they don't do an abortion and they don't do adoption. They chose to have the baby.

Then, when the demands of the baby / infant / child becomes too much, the torture they inflict upon that child is nothing short of heinous. From stomping a child to death for wetting their pants, to starving the child, making the child stand for hours in one spot holding a heavy object, and on and on.

I don't know if mankind is getting more violent towards their offspring or the media is simply bringing these cases to the forefront, but it's the most inhumane thing to read about when a child is tortured and killed by their parent(s) / guardians.