I'll write two possible scenarios:

First one: The troubled man was in a hunt for all the people that haunted and assaulted his family. He had an emotionless face expression because at the same time his dark soul had all the possible emotions: he just found the head of the organization that murdered his wife and raped their children in an operation to "wipe off" all the people in a village that was rich in oil.

The shard of glass that was used was the same one that it was used to kill his wife. He found her wife in agony when he came back home from a revolt in the village, she had that piece of glass buried in her heart. Her last words before passing away were: "I want you to know that I love you, and maybe my heart is dying, but i hope you remember me and carry a piece of my heart with you". He took out the glass off her chest, as an interpretation of his wife's words, buried his wife and children, and he made a plan to avenge the deaths. All this man's killings were made with this same "weapon".

Fast forward. After killing the man, the troubled man just felt he achieved his vengeance, he did not care what would happen next. So he just waited for the police with a sense of closure, drinking the beer.

Second One: This man was a twisted version of Sheldon Cooper (The Big Bang Theory) and he said in his mind: "He's in my spot".