His friend insisted that he must, but his conscience stopped him as he knew that he what?
Options:
- unnecessary
- optional
- will not
- need not
- should not
Let's think carefully first. Step-by-step reasoning process:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
Should is not is used when one wants to stop. Should not is used in contrary to must. So the final answer is should not.