When the Torah mentions the prohibition of joining with a false witness (Shemot 23:1), it does so with a very suggestive phrasing – it says not to do so and “become a violent witness.” The word, “hamas,” which we have translated as violent, essentially involves the undue human pressure on a process not meant to be under someone’s control. The classic case is when one tries to intimidate someone else to sell something they don’t want to sell. The sale is a permissible act but it is precipitated via inappropriate methods.
If someone knows that another person is guilty of a crime (since they saw the person in the act) but can only bring them to justice by teaming up with a false witness, the person joining with the false witness is resorting to a form of unjustified pressure. The ends definitely do not justify the means.
The Torah is actually telling us a bit more by using the word “hamas.” The Torah is pointing out here that a person could be testifying the complete and honest truth. Yet if he joins in testimony with another witness that he knows is likely to lie, he not only does something wrong but changes his very identity. In other words, to be involved in such an association has the potential to do violence to your own character. It can turn someone into a completely different person.
The rabbis took this idea even further. On this verse, the Midrash (Mechilta, Mishpatim 196) comments that the dignitaries of Jerusalem would first inquire about the guest list before coming to a party. If the list included people whose association would compromise them, they declined the invitation. They understood that a man is very much defined by who his friends are. And when it comes to a person’s essence, one must be careful.
This is not to say that they would avoid such people in the street. Indeed, the rabbis point out how one should greet everyone with a friendly demeanor. Rather, the problem only begins when two or more people join in a common purpose. And though not obvious, a party also involves a type of common purpose.
We can’t choose all the influences and settings that form our identity. But there are things we can choose. About those things we must remember what is at stake.