Charles Ellicott Commentary 1 Chronicles 10:5

"And when his armor-bearer saw that Saul was dead, he likewise fell upon his sword, and died." — 1 Chronicles 10:5 (ASV)

He also fell on the sword. —Samuel says, “his sword,” that is, the sword of the armor-bearer.

And died. —Samuel adds “with him,” which seems to be omitted here for brevity. This may also be the reason for other similar omissions. Loyalty to his chief, and perhaps dread of the enemy, were the armor-bearer’s motives.