Shaw's background is working for the Nazis during WW2. He observes to Eric that their methods work.

> The one thing I can say for the Nazis is their
methods seem to produce results.

Shaw then puts Eric's mother in mortal danger.

> Here's what we're going to do.
I'm going to count to three…
…and you're going to move the coin.
You don't move the coin, I pull the trigger.

He then shoots Eric's mother producing the desired result of energising Eric's gift:

> Understand?[ ..............]
So we unlock your gift with anger.
Anger and pain.
>You and me, we're going to have a lot of fun together.

[From Script](https://www.springfieldspringfield.co.uk/movie_script.php?movie=x-men-first-class)

Shaw asks Darwin what his ability is:

> I adapt to survive.

When the crunch time comes and they face off, Shaw easily absorbs the energy from Darwin's punch, leaving Darwin nonplussed for a moment.

Shaw recognises and acknowledges Darwin's ability, and gives him a chance to prove himself:

> Adapt to this.

and pops an energy-ball into Darwin's mouth without apparent rancour.

Shaw uses the tried and tested method to unlock the gift as before. The fact it doesn't have sufficient effect would seem to just be the outcome of natural selection - quite in line with Darwin's name, the method being nothing beyond what the Nazis found to be effective. 

Darwin is just collateral damage, one of the failed experiments. An evolutionary dead-end. Fun over.