Why?
Probably because he just wanted to know what it was like to kill someone. He wanted to know what it would be like to enact upon his power to destroy the lives of others simply out of curiosity.
As he saw the ways in which his family members talked to others with joking insults, (which he says that he didn't like, but didn't exactly hate) that curiosity was constantly rekindling and festering until he finally decided to pull the trigger.
As for why he didn't run away?
He felt no need to. He had planned for months that this event was going to be the climax of his life. He probably felt no urge to continue doing anything else in society and so, after he accomplished his goal, he saw no reason to attempt to flee.
Probably because he just wanted to know what it was like to kill someone. He wanted to know what it would be like to enact upon his power to destroy the lives of others simply out of curiosity.
As he saw the ways in which his family members talked to others with joking insults, (which he says that he didn't like, but didn't exactly hate) that curiosity was constantly rekindling and festering until he finally decided to pull the trigger.
As for why he didn't run away?
He felt no need to. He had planned for months that this event was going to be the climax of his life. He probably felt no urge to continue doing anything else in society and so, after he accomplished his goal, he saw no reason to attempt to flee.