The sentence is mostly understandable but not quite grammatical. It should be rephrased in one of two general ways:
- You don't need so righteous a man as that.
- You don't need so righteous a man to do that.
The first version is making a comparison between a level of righteousness and a particular man, while the second version is comparing a level of righteousness with the needs of a particular task.
When you combine both (a man as to do that) it's unclear which of the two senses is actually meant.
If you want to maintain a so . . . as construction but use it in the second sense, you could rephrase it:
You don't need a man so righteous as that in order to accomplish the task.
You don't need a man so righteous as that for the job.
Note that there is a subtle difference in meaning between those two, even though they are broadly the same. The first says that the task can be completed by somebody less righteous, while the second can additionally be inferred to say that somebody so righteous should not be used for the job.