What happen to my piston?

Hey guys little back story. Bought this car off an old man for 1200 bucks. Said he bought it from his wife’s coworker who was hurting for cash. Had for 2 years and was selling the rental house where he had the car parked. So the car had to go. Man said he replaced the carb and battery and it started up. I drove it about five miles it was fine. Good power. For what it is. Sounded good.
Took it home. Drove it again about a week later to a nearby town and back. 30 miles round trip. About a half mile from my house on the way back the motor was badly off balance and kept wanting to die unless I was punching it. Make a weird loud maybe squeal like sound if I recall correctly. Started it back up and punched it all the way and parked it. Just had the chance to move and now have a place to work on the car and I tore the block down. I found three things that were out of place or odd. The oil pump shaft from the distributor was snapped. Piston number 4 and corresponding on the block was damaged. Stuffed up severely. I’m entirely new to all of this and have decided to go with an explorer 5.0 build but am curious as what would cause the piston to do that? Snapped shaft and no oil getting there?
Any input would be greatly appreciated.