Like I said, here are a few lines on NEED VS MUST.
NEED can be used as an ordinary verb and as a modal verb. The modal verb does not change its tense. It is most common to use the negative.
You need not worry, you will be all right.
You need not speak so loudly.
I need a drink.
So NEED usually, please correct me if I am mistaken, talks about whether or not there is a need or reason to do what you are doing. The negative, as said, being common, can give permission not do to something and can say that there is little reason to do like you are doing.
You need not whisper. You need not cut the grass.
However, MUST and NEED can be similar in the following sentences:
I must meet Susanne.
I need to meet Susanne.
I would put forward, yet to be discussed, that NEED has notion of the speaker deciding, whereas MUST, as well as HAVE TO talk about a, most likely external, obligation.
What do you think?