A human has roughly over 6000 thought a day. They can not possibly all be either positive or neutral.
Negative thoughts are ok, one should not deny them from themselves or others. It's about how one addresses negative thoughts and how one moves or acts on them.
It is beneficial to pause and observe why one feels like this if negative thoughts take control of one's mind.
If it is something one can have an influence on, act on it. If it is something one can not have an influence on, take note on the situation and let it pass.