Because sometimes I can’t think of pithy titles.
Assassin showed up in the mid sixteenth century, coming to English via French and Italian but originally from Arabic, where it was hashishiyyin. That word actually meant “hashish users”, because apparently during the Crusades there was a sect of people who got high on hashish and murdered the opposition. Not really assassinate, though, so it’s weird that we gave it a different connotation. Fun fact, Middle English actually had it as the word hassais, which in terms of pronunciation is definitely closer to the original Arabic.
Slaughter is really just laughter with an S on the front. Anyway, it showed up in the fourteenth century as a word that could mean the killing of cattle/sheep for food or the killing of a person. It comes from the Scandinavian word slahtr, which is close to the Old Norse slatr, butchering meat. That’s from the Proto Germanic slukhtis, which is related to sla, the origin word for slay, from the Proto Indo European slak-, to strike. Well, it’s hard to kill something without striking it. Hard, but not impossible.
Injury showed up in the late fourteenth century, while just injure showed up in the mid fifteenth century. It’s from the Anglo French injurie, wrongful action, and related to the OldFrench injuriier, damage or offend, and before that the classical Latin [http://omniglot.com/writing/latin2.htm] iniuria, which could mean injury as well as a wrong or injustice. It’s made up of the prefix in-, opposite of, and iurius comes from ius, right (like a right, not the direction :P). An injury is an opposite of a right… right?
Maim showed up in the fourteenth century from the Old French mahaignier, wound, mutilate, cripple, or disarm. So pretty much what we use it as today. It’s thought to be from the Vulgar Latin mahanare, but it’s not certain and anywhere else it may be from (like related to mad) is even more speculative.
Attack is the most recent of these words, having not shown up until the seventeenth century. It’s from the French (that’s modern French) attaquer, which in turn is from Florentine Italian (a dialect spoke specifically in Florence) attaccare battaglia, join battle. It’s actually related to the word attach, but weirdly enough not with the affix definition we use with it, which is from somewhere else entirely.
Words are very confusing sometimes. A lot of the time.