For a long time I didn’t understand why the Roman soldier stabbed Jesus with a spear after He was dead. Then I learned as executioners, the soldiers were responsible to make sure their charges died. If they lived, the soldiers would be crucified. That explained that.
The professional soldier would know exactly where the heart was and how to pierce it from below. A medical doctor determined Jesus likely died of congestive heart failure from dehydration due to flogging and blood loss. The blood and fluid gathered around His heart and poured out when pierced. Fulfilling prophecy, too.
The flagellation wounds on His back continued bleeding, never coagulating, because He continually rubbed them against the rough cross as He heaved Himself upward again-and-again to draw each breath. Thus, His scourging’s unusual severity actually hastened His death; crucifixion’s normal duration was two-three days, which was part of the genius of it in Roman eyes: agony extended. That is why they were surprised He was already dead.