We’ll never know. Maybe he saw red and lost his mind.
What do know is that 99% of defenders let up when a QB slides and don’t forearm shiver their heads into the turf.
Conflating this with the bang bang “defender doesn’t have enough time to completely avoid contact” plays is just excusing bad behavior. And you’re wrong. Which is why virtually all NFL players, media, coaches all disagree with you. Despite what the “game was like” with black and white television.