Well, being a grammar nazi doesn't always make you a jerk, but constantly correcting people's grammar (or typo) can be considered as harsh and insulting. Hence, a grammar nazi who feels the NEEDS of correcting people's grammar should learn how to do that in a correct manner, politely.
For me, the most important trait that a grammar nazi must have is the ability to correct him/herself when making mistakes. Ability to sense the own-mistakes and naturally fix them are substantial. Somehow, the mistakes being made are awakening the feeling of "nobody is perfect" or "anybody can make mistake". Thus, if a grammar nazi has a strong sensibility in criticizing him/herself, he/she will also understand the feeling of being "knocked down" or belittled by others for grammatical/typo mistakes he/she is
Honestly, I have a strong sense to "embarrass" people when it comes to grammatical/typo mistakes they made #d'oh. How? By bluntly pinpointing their mistakes #grin. That's my major weakness that I completely realized. Over this past 5 years, I have that weakness 95% cured with a simple recipe. And that recipe is to find mistakes/typo in the books, magazines, newspapers or any printed papers. So I'll just crossing out every grammatical mistakes/typo I found on the printed paper I currently read and fix them. Hence, almost all my books are "beautifully accessorized" with scratches and comments #tongue. I think this is the best way that I can do to hamper the fun of embarrassing others.