Leeches are segmented parasitic or predatory worms that comprise the subclass Hirudinea within the phylum Annelida. They are closely related to the oligochaetes, which include the earthworm, and like them have soft, muscular segmented bodies that can lengthen and contract. Both groups are hermaphrodites and have a clitellum, but leeches typically differ from the oligochaetes in having suckers at both ends and in having ring markings that do not correspond with their internal segmentation. The body is muscular and relatively solid, and the coelom, the spacious body cavity found in other annelids, is reduced to small channels. The majority of leeches live in freshwater habitats, while some species can be found in terrestrial or marine environments. The best-known species, such as the medicinal leech, Hirudo medicinalis, are hematophagous, attaching themselves to a host with a sucker and feeding on blood, having first secreted the peptide hirudin to prevent the blood from clotting. The jaws used to pierce the skin are replaced in other species by a proboscis which is pushed into the skin. A minority of leech species are predatory, mostly preying on small invertebrates. The eggs are enclosed in a cocoon, which in aquatic species is usually attached to an underwater surface; members of one family, Glossiphoniidae, exhibit parental care, the eggs being brooded by the parent. In terrestrial species, the cocoon is often concealed under a log, in a crevice or buried in damp soil.
This may be considered a part of the defense of provocation against charges of murder. Based on the idea that all individuals may suddenly and unexpectedly lose control when words are spoken or events occur, jurisdictions differ on whether this should be allowed to excuse liability or merely mitigate to a lesser offense such as manslaughter, and under which circumstances this defense can be used. In many common law jurisdictions, provocation is a partial defense that converts what would have been murder into manslaughter. A few jurisdictions do not prosecute (Iran, Iraq) or have a lesser penalty (Kuwait, Egypt) for honor killings. European Convention on Human Rights
Article 2 Paragraph 2 of the European Convention On Human Rights provides that death resulted from defending oneself or others, arresting a suspect or fugitive, or suppressing riots or insurrections, will not contravene the Article when the use of force involved is "no more than absolutely necessary":

2. Deprivation of life shall not be regarded as inflicted in contravention of this Article when it results from the use of force which is no more than absolutely necessary:
(a) in defence of any person from unlawful violence;
(b) in order to effect a lawful arrest or to prevent the escape of a person lawfully detained;
(c) in action lawfully taken for the purpose of quelling a riot or insurrection.