Not a happy ending!

Having just rewatched The Woman in Black, there's no doubt in my mind that the ending was not a 'mercy killing' or an act of 'kindness' from the Woman in Black to Arthur.

Take a look at the scene where he's delivered her sons corpse to her room.

https://www.youtube.com/watch?v=PMMkc-LlOn8

At around 00:17 seconds we see The Woman in Black reach down to hold her son, if you listen closely you can hear his spirit shouting back;

"NO! YOU'RE NOT MY MOTHER!"

This leads me to the fact that The Woman in Black would no doubt have been outraged and utterly destroyed by her son saying this to her. Which references the next time we see/hear her and she's saying "never forget, never forgive" over and over. Because in her mind, her sister robbed her son of his true mother in life for her own selfish reasons.

So for the Woman In Black to attempt to kill Arthurs son was showing the audience that she will never change, because the on thing she wanted in this world and the next rejected her.