There are two accounts Saul's death in the Old Testament. Both say he committed suicide (I Samuel 31:4-5 and I Chronicles 10:14) but the latter one also says that: "the LORD killed Saul because ..." Why the difference?
"So Saul died for his unfaithfulness which he had committed against the LORD, because he did not keep the word of the LORD, and also because he consulted a medium for guidance. But he did not inquire of the LORD; therefore He killed him, and turned the kingdom over to David the son of Jesse" (I Chronicles 10:13-14).
The works of I Samuel through II Kings tells us of the events that happened in Israel's history. The focus is more on what happened. I and II Chronicles looks at the same events and gives us deeper insight as to why things happened as they did.
Saul had several incidences where he did things his own way instead of following God's commands (I Samuel 13:13; 15:22-28). This eventually led to God no longer guiding him. "And when Saul inquired of the LORD, the LORD did not answer him, either by dreams or by Urim or by the prophets" (I Samuel 28:6). Saul thought he could sneak past God by finding a medium to talk with Samuel who was by this time dead. Instead of waiting for God, Saul once again tried to force things to go according to his views and his schedule. Thus, we are told why Saul lost the Philistines, while David fighting on another front defeated the Amalekites.
Saul was mortally wounded by an arrow during the battle with the Philistines (I Samuel 13:3), but Saul could not stand the thought of allowing the Philistines a chance to brag that they had killed him, so he ordered his armorbearer to kill him. The man refused, so Saul fell on his own sword. Saul was going to die in this battle. God had already told him that. Once again, Saul tried to force the situation where he had control and did things on his schedule. What he never realized is that he changed nothing.
Thus, Saul did commit suicide. But it is also true that God slew him because God made sure that Saul lost the battle with the Philistines. God didn't make Saul commit suicide, that was Saul's own foolish idea of trying to control what could not be controlled.