Pretty common to have that "crack" sound. I was afraid perhaps you were trying to remove the nut with the Lock tab washer still in place. Not good!
Be sure to buy a new lock washer for re-installation. You can use old one but new one is better. (and cheap)
Next time you encounter a big nut or bolt like this try the following to make it break loose easier:
1. Soak with WD40 or other penetrating oil. Let sit a couple days.
2. Heat lightly and gradually. Not too hot as you have a rubber seal in there, but some heat is OK.
3. Strike sharply with hammer .... very carefully onto nut, not the shaft.
Not too hard, just a few firm shots.
All these things can ease the breaking of a stubborn high torque nut.
Next time, keep an eye on your chain and sprockets for wear. To give much longer chain life to your new chain I recommend replacing your countershaft sprocket at about 10K to 12K miles irregardless of how you think it looks. Just change it. Do it sooner if doing a lot of mud riding.
A worn countershaft sprocket wears the chain faster than anything else. Use stock Kawasaki OEM sprockets, not BS aftermarket crap. (JT and AFAM are not too bad, but stock is still better value)
Keep chain clean and lightly oiled. No oil needed when in dirt. But after dirt riding, clean and re-oil for street riding.