Continuity mistake: After king Willie has had his head cut off, the marks on his cheeks are much darker and drawn on with marker pen. Otherwise, the decapitated head is quite convincing.
Continuity mistake: When the Columbians are in the armoury near the beginning, they all look up as a skylight shatters. In the slow motion shots, you can see the leader turn away from the glass. But, then it shows him in a closer shot, and he's still facing the falling glass, and falling back away from it, shielding his face with his hands.
Answer: As a reward for besting the Predator in combat, he proved himself a fierce warrior worthy of praise and the gun was a token of their respect for his strength.