(covers information from several alternate timelines)
Homicide was simply the act of one individual causing death of another, either intentionally or unintentionally. Classifications of homicide include accidental death, assassination, capital punishment, casualty of war, culpable negligence, euthanasia, or murder.
T'Pol likened the crew of the Seleya, who had been exposed to trellium-D and had their ability to repress emotions stripped away, to those early pre-Surak Vulcans, who regularly experiences paranoia and homicidal rage. (ENT: "Impulse") T'Pol later explained her experience with trellium-D aboard the 'Seleya with Doctor Phlox, when she too was "homicidal and paranoid", confessing that " initial effects were overwhelming, but as they began to wear off I discovered I was able to access certain emotions," causing her the experience the desire of wanting to experience more, eventually leading to an addiction to the deadly compound. (ENT: "Damage")
In 2258 of the alternate reality, upon James T. Kirk's awakening following a transfusion of Khan Noonien Singh's blood, Leonard McCoy questioned whether Kirk was feeling homicidal, power-mad or despotic. Kirk replied, "No more than usual." (Star Trek Into Darkness)
Ira Graves bragged of his success as a genius to Data because he possessed "an iron will and nerves of steel;" adding "as well as the lady killer I was in days gone by." Upon hearing this revelation, Data misunderstood the expression, inquiring if Graves condoned homicide, before having it explained that Grave's considered himself to be "once as beautiful as [he was] smart." (TNG: "The Schizoid Man")
While portraying the role of Sheriff in Alexander's Ancient West holoprogram in 2369, Worf confronted the main antagonist, Eli Hollander, with the intent of arresting him, stating, "You have been accused of committing a homicide." When confronted, Hollander looked up at Worf and confessed, "If you mean murder, then I guess I'm guilty. I've killed twenty three men, Sheriff." Hollander then threatened Worf by adding, "One more ain't going to make much of a difference. Of course, shooting someone as ugly as you couldn't be considered a crime, could it? Was you just born that way, or did your momma marry an armadillo?" (TNG: "A Fistful of Datas")
In 2369, Commander Benjamin Sisko faced the extradition of the Rakhari criminal Croden to his homeworld, Rakhar, for the homicide of Ro-Kel, an event that coincided with making first contact with the planet. While discussing this prospect, Miles O'Brien felt that the circumstances were "not the best way of introducing ourselves," with Bashir, who shared O'Brien's sentiments, adding, "[h]omicide as a means to open relations, it's not exactly how they teach it at the Academy, is it." (DS9: "Vortex")
Following the death of former Bajoran collaborator Ches'sarro Seeto in 2370, Odo asked if Major Kira Nerys could advise the medical examiner that he wanted a complete autopsy on the victim, adding that "his death is to be treated as a suspected homicide." (DS9: "Necessary Evil")
Later that year, when Odo discovered that Elim Garak was a former member of the Obsidian Order, he requested that Doctor Bashir allow his to talk to the former spy as soon as possible because he had four cases left in his homicide files the he was "almost certain were committed by the Obsidian Order." Odo then explained that being able to discuss this with Garak might be able to help shed some light on them. (DS9: "The Wire")
When Quark learned that Odo was reading a crime novel, Odo, who described his reading as "work, not recreation", explained that he did so because "books like this give me insight into the criminal mind." Odo then added that, "[i]t is a well known fact that a high percentage of homicides are motivated by romantic obsession." (DS9: "The Ascent")
After Gul Dukat was released by the Pah-wraith who was possessing him when he killed Jadzia Dax, he stood over Dax's corpse and told spoke that "I know this is small comfort, but I never intended you any harm." (DS9: "Tears of the Prophets")
When USS Voyager visited the Mari homeworld, B'Elanna Torres was arrested for attacking a victim in response to a violent thought, however, because the Mari were a mostly non-violent people, the planet's Chief Examiner Nimira was ill-equipped to handle such a case. As such, Nimira requested the help of Voyager's chief of security, Lieutenant Tuvok, explaining that she had "never dealt with a murder before" and that he, on the other hand, "had experience with homicide investigations." Nimira, nonetheless, though the proper course of action would be to trace the whereabouts of "the old woman" for the past few days to see where she might have encountered the thought, a tactic Tuvok described as a "wise choice". (VOY: "Random Thoughts")
When Silver Blood Tom Paris was about to marry Silver Blood B'Elanna Torres, the crew expressed their disbelief that they would have ever gone through with the process, with Silver Blood Seven of Nine observed that "Given the volatile nature of their relationship, one might have predicted homicide rather than matrimony." (VOY: "Course: Oblivion")
Many instances of the various homicides (and some murders) in Star Trek can be defined in more specific terms than were used on screen, namely several instances of that would typically be considered as manslaughter. More specifically, Odo's reference to what was essentially a crime of passion, Croden's unplanned murder of Ro-Kel, and Dukat's killing of Jadzia Dax being prime examples of this form of homicide.