Several things I noticed in these pictures.
1) Saul isn’t portrayed his actual age. He ruled 40 years and was 70 when he died.
2) David was 30 at the time. He ruled 7 years in Hebron before he became king over all Israel. I don’t think all the paintings show David as 30.
1 Kings 2:11
The period that David reigned over Israel was forty years; seven years he reigned in Hebron, and in Jerusalem he reigned thirty-three years.
3. I have not considered Saul may have stabbed himself in his chest/heart. I always assumed it was the stomach.
4. The extreme brutality of these fighting men. David said, ‘Kill him’ and the Amalekite died within the minute. I knew warriors became superbly talented with their spears and swords (and slings and bows) but the quickness in killing is hard to imagine.
I assumed the heart, with an upward thrust up under his ribs, because the text indicates his purpose was to avoid enemy abuse. Thus, he wanted to ensure his own quick death, without lingering: heart=instantly, stomach=bleed to death comparatively slowly.
4. the quickness in killing is hard to imagine.
Slice the neck, eliminating blood supply to the brain and effectively decapitating: twenty seconds.