In the bhagavad-gita the primary story concerns a war between two forces, good versus evil. nevertheless, arjuna’s concern about participating has to do with:

Bhagavad-Gita primary story is a concern with the war between two forces good versus evil in spite of that  Arjuna's was concern about participating is because he had to fight with his own brothers(Kaurav), teacher(Dronacharya) and grandfather (Bhishma Pitamah).

He was also concerned because this war will lead to the death of his loved once.

Krishna told Arjuna that those who are on the side of Evil are also Evil irrespective of the relation so, to protect the world he should fight.