1)
Why did he curse David?
1.
Malbim: Initially he thought that each side will choose a representative [and the nation of the defeated will serve the victor - refer to 17:9:1:1 and the note there]. For this, the Kelim must be the same. When he saw that David has no weapon, he realized 1 that he merely comes for vengeance and to disgrace him.
2)
Why did he say "ha'Kelev Anochi"?