SAUL DIED BY HIS OWN SWORD.

1 Samuel 31 says that Saul was injured in battle and then he used his own sword and fell on it. We read, "The fighting grew fierce around Saul, and when the archers overtook him, they wounded him critically. Saul said to his armor-bearer, 'Draw your sword and run me through, or these uncircumcised fellows will come and run me through and abuse me.'  But his armor-bearer was terrified and would not do it; so Saul took his own sword and fell on it. When the armor-bearer saw that Saul was dead, he too fell on his sword and died with him. So Saul and his 3 sons and his armor-bearer and all his men died together that same day."  (1 Samuel 31: 3-6)
1 Chronicles 10 says: "Now the Philistines fought against Israel; the Israelites fled before them, and many fell slain on Mount Gilboa. The Philistines pressed hard after Saul and his sons, and they killed his sons Jon'athan, Abin'adab and Malki-Shua. The fighting grew fierce around Saul, and when the archers overtook him, they wounded him. Saul said to his armor-bearer, 'Draw your sword and run me through, or these uncircumcised fellows will come and abuse me.' But his armor-bearer was terrified and would not do it; so Saul took his own sword and fell on it. When the armor-bearer saw that Saul was dead, he too fell on his sword and died. So Saul and his 3 sons, and all his house died together."
(1 Chronicles 10: 1-6)
The saying "live by the sword and died by the sword" is an idiom that basically means "what goes around comes around." If anyone use violent, forceful, or underhanded methods against other people then that individual can expect those same methods to be used against his body."
The saying has become a common expression, adapted from a conversation between Jesus and His disciple Peter just before Jesus was arrested in the Garden of Gethsemane. After Jesus was betrayed by the only Judean of the 12, Judas Iscariot, a group of soldiers moved in to arrest the Lord. In a rash attempt to protect Jesus, Peter pulled out his sword and struck the servant of the High Priest, cutting off his ear (John 18:10). Jesus rebuked Peter and put a quick stop to the bloodshed. Jesus replaced the wounded man's ear, healing him instantly (Luke 22:51). Then Jesus told Peter to put his sword away, for "all who draw the sword by themselves will die by the sword" (Matthew 26:52). Jesus also told Peter that He himself would not fight the arrest, for it was God's Will that He drinks from the cup that was given to Him (John 18:11).
In Saul's case, the figurative usage of "falling on one's sword" is when someone takes personal responsibility for a group action.
The expression also is the Anglicized equivalent of "hara-kiri" -Japanese samurai custom of committing suicide by disembowelment (belly-cut) with a sword rather than face the dishonor of surrender.
The way in which Judas Iscariot (the only one from Judah), died give us a better comprehension about the spiritual level in which Saul found himself that ended the life of his whole house.
Acts 1 says: "In those days Peter stood up among the believers (a group numbering about 120) and said, 'Brothers, the Scripture had to be fulfilled which the Holy Spirit spoke long ago through the mouth of David concerning Judas, who served as guide for those who arrested Jesus -Judas was one of our number and shared in the ministry.' (With the reward he got for his wickedness, Judas bought a field; there he fell headlong, his body burst open and all his intestines spilled out. Everyone in Jerusalem heard about this, so they called that field in their own language 'Alkedama,' that is, 'Field of Blood'). "
"For" said Peter, "It is written in the Book of Psalms, 'May another take his place of leadership.' Therefore it is necessary to choose one of the men who have been with us the whole time the Lord Jesus went in and out among us, beginning from John's baptism to the time when Jesus was taken up from us. For one of these must become a witness with us of His resurrection" (Acts 1: 15-22).
Samuel was chosen and anointed by the prophet Samuel to be king of Israel, not because God wanted to, it happened because the people of Israel wanted a king like their neighboring nations. In the ceremony Samuel warned Saul and the nation of the need to follow God and not depart from His instructions. Saul won a couple of great battles and went from being humble to being proud. Under instruction from God Saul led a battle against the Amelekites. This was because Amelek ambushed Moses during the Exodus. The instruction was to kill everything and everyone. Saul didn't do that but instead spared the king and the best of the animals. He then lied when Samuel asked if he obeyed. When challenged by Samuel he then blamed the people. Samuel the pronounced the judgment from God: "Has 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 Heed than the fat of rams. For Rebellion is as the Sin of Witchcraft, and Stubbornness is as Iniquity and Idolatry."
When Judas betrayed Jesus to the assembly of 23 or 71 rabbis appointed (Sanhedrin) to sit as a tribunal in the Garden of Gethsemane, at the foot of the Mount Olives in Jerusalem (by kissing him and addressing him as "Rabbi" to reveal His identity to the crowd who had come to arrest Him) he received 30 pieces of silver. The Gospel of Luke 22:3 and the Gospel of John 13:27 suggest that he was possessed by Satan. Matthew 27: 1-10 says that Judas after learning that Jesus was to be crucified, he attempted to return the money he had been paid for his betrayal to the chief priests and committed suicide by hanging. The priests used the money to buy a field which was called the "Field of Blood" because it had been bought with blood money.
The same series of events happened to Saul. He disobeyed God's command, he died by his own sword, and his complete house ended with him, showing us the real meaning of Disobedience to God. The action of Rebellion is treated as if it is a Sin of Witchcraft , Stubborness as if it is Iniquity and Idolatry.
In 2 Samuel 1, the Amalekite's claim to have killed Saul is based on the Judgment Saul faced when he disobeyed God's command and because of his stubbornness and rebellion towards God he and his house was sentenced to death.