Shouldn't it be negative since it shares one of Nitrogen's free electron pair with an O to form a double bond?
The nitrogen has five valence electrons. Here, only four are involved in the covalent binding of the -O, =O, and -OH. Only one of the two electrons involved in the covalent bonds belongs to the nitrogen. As there are only four covalente bonds, nitrogen is lacking one electron, therefore it is positively charged.