We never actually meet Selden as a character, but he's clearly supposed to catch our attention as a possible threat to Sir Henry and to the neighborhood in general. After all, he's an escaped convict from the famous prison of Princetown at Dartmoor. He's been convicted of such a brutal murder that he escaped the death penalty on an insanity plea (6.38). And when Watson catches a glimpse of Selden out on the moors, he is struck by his "terrible animal face, all seamed and scored with vile passions" (9.126).
Clearly, Selden is a nasty piece of work who wouldn't hesitate to do terrible things to the people of Baskerville Hall if he could. When he falls to his death running away from the Hound out on the moors, everyone seems quietly glad that he is gone.
All these descriptions of Selden don't jive with Mrs. Barrymore's version of the man once it is revealed that her maiden name is Selden, and that the convict Selden is also her baby brother. She tells Sir Henry of the night that "he dragged himself here […] weary and starving, with the warders hard at his heels" (9.74).
Maybe we aren't supposed to feel pity for Selden, but we definitely feel terrible for Mrs. Barrymore, who looks at this hunted murderer and sees her little brother. In fact, Watson reflects when he breaks the news of Selden's death out on the moors to the Barrymores:
To all the world he was the man of violence, half animal and half demon; but to her he always remained the little wilful boy of her own girlhood, the child who had clung to her hand. Evil indeed is the man who has not one woman to mourn him. (13.21)
Here, following Selden's death, Watson thinks about what it really means that even the worst of criminals are human, with families who suffer when they commit crimes.
In fact, this concept of the emotional damage done to families by crime is a theme in The Hound of the Baskervilles as a whole: not only does Stapleton abuse his wife, Beryl, but he is technically the nephew of his first murder victim, Sir Charles, and he attempts to kill his cousin by blood, Sir Henry. Although the ties of family are very strong for the Barrymores and the Baskervilles, family's also no protection against either crime or pain in this novel.