Charles Spurgeon Commentary John 19:32-34

"The soldiers therefore came, and brake the legs of the first, and of the other that was crucified with him: but when they came to Jesus, and saw that he was dead already, they brake not his legs: howbeit one of the soldiers with a spear pierced his side, and straightway there came out blood and water." — John 19:32-34 (ASV)

Then came the soldier, and brake the legs of the first, and of the other which was crucified with him. But when they came to Jesus, and saw that he was dead already, they brake not his legs: but one of the soldiers with a spear pierced his side.

To make sure that he should not survive.