Who is Gasemu?Why does Ishmael befriend him and then later try to strangle him?

Gasemu is a man from Mattru Jong. He knows Ishmael and his family. Ishmael tries to strangle Gasemu after he detains the boys from entering the village, which is subsequently attacked by rebel forces. Beah blames Gasemu because he misses the chance to see his family, as they are all killed during the attack.