What does Stanley find out about Zigzag in Chapter 17 of Holes by Louis Sachar?

Stanley learns Zigzag has harbored a grudge and can be violent. 

All of the boys at Camp Green Lake go by nicknames.  The nicknames are generally not flattering.  Stanley’s nickname is Caveman.  Other boys are named Zigzag and Zero.  Zigzag thinks Zero is weird, but Stanley thinks Zigzag is pretty weird, too:

Zigzag had to be the "weirdest dude" Stanley had ever seen. He had a long skinny neck, and a big round head...

Stanley learns Zigzag has harbored a grudge and can be violent. 

All of the boys at Camp Green Lake go by nicknames.  The nicknames are generally not flattering.  Stanley’s nickname is Caveman.  Other boys are named Zigzag and Zero.  Zigzag thinks Zero is weird, but Stanley thinks Zigzag is pretty weird, too:

Zigzag had to be the "weirdest dude" Stanley had ever seen. He had a long skinny neck, and a big round head with wild frizzy blond hair that stuck out in all directions. His head seemed to bob up and down on his neck, like it was on a spring (Ch. 7).

It turns out Zigzag has some violent tendencies.  All of the boys are forced to dig holes all day long that are as wide and deep as their shovel.  It is supposedly character-building, but the boys know they are looking for something.  The boy who finds something when digging is allowed the rest of the day off.  Zigzag is upset when Stanley finds a lipstick tube but gives it to X-ray instead of him, and expresses his frustration violently: 

[Stanley] scooped up some dirt, and was raising it up to the surface when Zigzag's shovel caught him in the side of the head.

He collapsed.

He wasn't sure if he passed out or not. He looked up to see Zigzag's wild head staring down at him. "I ain't digging that dirt up," Zigzag said. "That's your dirt." (Ch. 17) 

Zigzag is obviously still angry at Stanley.  “Mom” makes no response except to tell Stanley not to nap.  This is not a camp where anyone cares what happens to the boys.  The boys only have value as diggers of holes to find treasure. 

It turns out the warden is a descendant of Kissing Kate Barlow, and she is looking for buried treasure.  She could not care less about the fate of the boys in her care.  They can bully and victimize each other all they want.  She is just as cruel.  Her only goal is the treasure.