1)
What is "Oyen"?
1.
Rashi: He had Ayin ha'Ra (envy of David). Radak - this is also according to the way it is written, without a Yud.
2.
Radak citing Targum Yonasan: He lied in ambush (for a chance to kill him).
3.
Malbim: He put David's Inyanim (matters) to his heart, and thought that he is "your colleague better than you" (15:28, to whom his kingship will be given). Therefore, he thought about how to kill him. 1
1
Shmuel told him that the decree cannot be changed! Perhaps due to Ru'ach Ra, Sha'ul overlooked this. Alternatively, because it is so hard to descend from authority (like R. Yehoshua ben Perachiyah said - Menachos 109b), he overlooked this. (PF)
