In The Stranger, what new knowledge do we gain about Mersault's character based on his interactions and relationships with Salamano and Raymond?

In , Mersault's interactions with Salamano and Raymond illustrate
Mersault's social indifference and refusal to question the meaning and motivation of other
people. 

Salamano is an old man who lives in Mersault's building. Salamano
regularly beats his dog. Mersault greets Salamano and asks him what the dog had done, but he
never really confronts Salamano's violent behaviour towards the dog. Mersault just accepts it as
the way things are. 

Mersault agrees to write a letter to Raymond's
girlfriend despite the fact that Raymond admitted to beating her. He even agrees to testify
against her on Raymond's behalf. Overhearing Raymond beating her again, Marie asks Mersault to
call the police. Mersault refuses, saying he doesn't like police.

When
Salamano's dog goes missing, Mersault observes Salamano's sadness and actually thinks of his
mother. This does indicate evidence of...