Well, crushing a guy's head is a bit over the top, but it does make his actions at the end much more surprising.
I think Ridley Scott did a great job with that film and I agree with the sentiment that Scott used Dick's novel more as a point of departure for a very different story.
"I've seen things you people wouldn't believe."
Well, it is explained by his wild despair over Tyrell’s revealing to him the secret of his short lifespan.
The fact is a madman like Tyrell wanted to make his creations immortal if he could but for various reasons (explained in that scene) he simply could not.