NUM309 Having relations with a [non-Jewish] maidservant is similar to the sins that incur death administered by beis din in two ways. [The first way is:] Our Sages z"l, said (Sanhedrin 81b), "A zealous person is permitted to kill one who has intercourse with a non-Jewish woman," as the pasuk says [this verse], "He speared both of them." This means that if one discovers the sinner going astray in public, before he has completed the sinful act--he may kill him the moment he confronts him. This is more severe than all of the offenses that incur death administered by beis din. This is because [in those cases] death can be administered only after the testimony of witnesses and due prior warning, and only by jurisdiction of the Sanhedrin; but this sinner may be killed by whoever finds him, without witnesses or prior warning. [This is also the opinion of Rambam (Hilchot Isurei Bee'ah 12:4); however, Ra'avad disagrees and requires prior warning, and so rules Rema (Shulchan Aruch, Choshen Mishpat 425:4)].