How does this happen?
How does a man go from being God's anointed and chosen man to lead Israel, to being dead, having his head placed in the temple of a foreign god, all of his sons dead and his lineage being stripped of the crown?
Saul committed suicide, but not on that fateful day when he fell upon his own sword.
He killed himself by doing several things. The first of them was his disobedience to God. When he was told specifically by God to "go and smite Amalek and utterly destroy all that they have and spare them not: but slay both man and woman, infant and suckling, ox and sheep, camel and ass", he disobeyed God's instructions and kept the best things for himself. If Saul felt it had value he did not follow God's word and instruction but his own selfish desire. This was the LORD's judgment on the Amalekites for they had attacked God's people in the wilderness when they were weak. (be very careful who you attack and when) and they were to be "utterly destroyed". Saul directly disobeyed the Word of the LORD. He thought he knew a "better way" than God and decided to do things his way instead of God's way.
Have no other gods before me.
How many times have we heard God's Word, or maybe even Him directly telling us what to do (God's way), yet in our disobedient selfish ways, we do something different (our way) and then wonder what happened and why we die. Does God ever say, "Have no other gods before me?" But how many times do we put money, cars, people and even our own prideful, lustful, vain self ahead of the very one who gave us the blessing in the first place?
It's this spirit of disobedience that causes our downfall, and ultimate death.
Remember the words of the LORD, "And Samuel said, Hath the LORD as great delight in burnt offerings and sacrifices, as in obeying the voice of the LORD? Behold, to obey is better than sacrifice and to hearken than the fat of rams." Disobedience is the first step in suicide. Disobedience leads to pride.
Scripture References:
1 Samuel 15 and 1 Samuel 31 (in case you are not familiar with the story).