Mersault´s stance towards two significant deaths

“The sun was the same as it had been the day I´d buried Maman and like then, my forehead especially was hurting me, all the veins in it throbbing under the skin.”(P.58-59)
Wow! From this citation I can profoundly connect the two most remarkable and imperative events which have risen the significance of the book until this moment. Two deaths have happened throughout the story, where the first one, Mersault´s mother one, made the spectators initiate to analyze and recognize the eccentric personality of the protagonist. After having understood the lack of emotions and sensitivity expressed from this character towards his mother’s death, it makes it easier for readers to understand Mearsult´s murder to the Arab, since he neither felt regret nor concern.
He just “...fired four times at the motionless body where the bullets lodged without leaving a trace”(P.59) That was it! Once more his emotional ignorance invades his mind!
Why did his forehead hurt just at the time when two deaths happened?... Would this be a warning sign to make him stop being so alienated from his feelings?