The formal prohibition of lo taguru does not apply in your case for a few reasons. First, it refers to dayanim, as is evident not only from the context of the pasuk, but also from the context in which it is cited in classical sources.[2] Although some sources extend lo taguru somewhat further,[3] it is too far of a stretch to apply it to require a lawyer to take a case, as a lawyer does not have a halachically formal part in the judicial process. Moreover, even in the case of a dayan, the prohibition applies only if he has heard the case to the extent that he has an idea regarding what the ruling should be.[4]
In general, it is problematic to assist a particular side in adjudication.[5] While there is an opinion that this warning is only to a dayan,[6] most poskim posit that no one should take sides without a valid reason.[7] What are grounds for taking sides? The gemara[8] says that it is proper to advise a litigant if he is a relative, invoking a pasuk[9] stressing the importance of helping relatives, as long as the advisor is not a prominent person. The Maharshal[10] applies this approach to helping a widow who is a litigant. Logic dictates that permission could also be given to help a litigant fight hardened criminals in court,[11] a task that the average individual cannot handle alone.
In cases in which giving advice is appropriate, is there an obligation or mitzva to help out as a lawyer? When the lawyer is (honestly) convinced that his client is correct, there should be a mitzva of hashavat aveida to help him win his case.[12] Thus, in the cases in which it is permitted to get involved, doing so should be included in that mitzva. However, the mitzva of hashavat aveida does not require one to put himself in a position of loss or hardship to save money for another.[13] This is all the more clear if there are any number of other people who can do the job, which makes the individual lawyer less specifically obligated than a person who spots a lost item.[14]
Returning to the case of the fearful dayan, the Shulchan Aruch[15] rules that a dayan who has a set public role is required to hear the case even when others would be able to refuse to hear the case out of fear. While the Radbaz[16] explains that this is because the public will help the dayan in such a case, the Beit Yosef[17] seems to understand that it is because someone who has responsibility cannot shirk it even in the face of reasonable concern. That logic would seem to apply to a lawyer with a role of district attorney, for example.[18] In a parallel case, the Tzitz Eliezer[19] allows and encourages a doctor to expose himself to patients with infectious diseases as part of his job. That being said, the job description of an average lawyer does not necessarily include angering dangerous criminals, and he therefore should not have to feel obligated to do so. If a lawyer wants to accept such a case anyway, there is generally permission for someone to put himself in at least moderate danger as part of his pursuit of livelihood.[20]
In summary, a lawyer need not feel an obligation to take on a case in which he will be up against a dangerous opposing litigant. He may choose to do so, preferably after discussing the matter with his family. This is a noble step if he has a unique opportunity to help someone who needs and deserves it.