If you follow the hanafi school, then it is Wajib. That means its a sin if you deliberately miss Witr.
The other scholarly opinions would regard it is a very important Sunnah. This is now broken down into 2 further opinions :
A) If you miss it its sinful.
B) If you repetitively miss it (ie. start making a habit of missing it deliberately) then that would accumulate to a Sin.
* I stand to be corrected on the other scholarly opinions.