This Bible contradiction is from the Skeptic's Annotated Bible.
Saul committed suicide.
1 Samuel 31:4-6 View context
Then said Saul unto his armourbearer, Draw thy sword, and thrust me through therewith; lest these uncircumcised come and thrust me through, and abuse me. But his armourbearer would not; for he was sore afraid. Therefore Saul took a sword, and fell upon it.
And when his armourbearer saw that Saul was dead, he fell likewise upon his sword, and died with him.
So Saul died, and his three sons, and his armourbearer, and all his men, that same day together.
1 Chronicles 10:4 View context
Then said Saul to his armourbearer, Draw thy sword, and thrust me through therewith; lest these uncircumcised come and abuse me. But his armourbearer would not; for he was sore afraid. So Saul took a sword, and fell upon it.
Saul was killed by an Amalekite.
2 Samuel 1:8-10 View context
And he said unto me, Who [art] thou? And I answered him, I [am] an Amalekite.
He said unto me again, Stand, I pray thee, upon me, and slay me: for anguish is come upon me, because my life [is] yet whole in me.
So I stood upon him, and slew him, because I was sure that he could not live after that he was fallen: and I took the crown that [was] upon his head, and the bracelet that [was] on his arm, and have brought them hither unto my lord.
Saul was killed by the Philistines.
2 Samuel 21:12 View context
And David went and took the bones of Saul and the bones of Jonathan his son from the men of Jabesh-gilead, which had stolen them from the street of Beth-shan, where the Philistines had hanged them, when the Philistines had slain Saul in Gilboa:
God killed him.
1 Chronicles 10:14 View context
And enquired not of the LORD: therefore he slew him, and turned the kingdom unto David the son of Jesse.