Izchak is a special shopkeeper who runs the lighting store in Minetown. He has a slightly different conversational repertoire from other shopkeepers, and has a fixed gender (like tool shop owners, but unlike other shopkeepers), but is otherwise identical for game purposes.
One of the founding members of the NetHack DevTeam was Izchak Miller. Professor Miller passed away on the 1st of April 1994 at the age of 58, from complications due to cancer. The DevTeam put Izchak the shopkeeper into the game as a tribute, inspired partly by his work on the shopkeeper code. The character on the navigation imagemap on the NetHack site is also named after him.
Although there are no special in-game penalties for doing so, many players consider it extremely poor form to kill this particular shopkeeper. The NetHack community itself is somewhat divided on this issue—in #nethack, it's rare to see someone reprimand another player for killing him (although most people try to avoid it); in RGRN, at least several vocal regulars will usually reprimand any transgressions; and the wiki itself sees a broad mix of editors' opinions.
For extinctionist games, it is considered acceptable to leave Izchak alive. (Note that although there is no "official" definition of extinctionism, most automated definitions, e.g. the ones used at the /dev/null/nethack tournament, care only about the monster kill and born counters and extinction flag. Getting these up to their maximum values for shopkeepers can be done via repeatedly killing any shopkeeper. Thus, you can play an extinctionist game while leaving Izchak alive by killing, resurrecting, and rekilling another shopkeeper to compensate; an extinctionist game would likely want to do this anyway to gain 120 or 255 shopkeeper kills.)
The shopkeeper of the lighting shop in the town level of the
gnomish mines is a tribute to Izchak Miller, a founding member
of the NetHack development team and a personal friend of a large
number of us. Izchak contributed greatly to the game, coding a
large amount of the shopkeep logic (hence the nature of the tribute)
as well as a good part of the alignment system, the prayer code and
the rewrite of "hell" in the 3.1 release. Izchak was a professor
of Philosophy, who taught at many respected institutions, including
MIT and Stanford, and who also worked, for a period of time, at
Xerox PARC. Izchak was the first "librarian" of the NetHack project,
and was a founding member of the DevTeam, joining in 1986 while he
was working at the University of Pennsylvania (hence our former
mailing list address). Until the 3.1.3 release, Izchak carefully
kept all of the code synchronized and arbitrated disputes between
members of the development teams. Izchak Miller passed away at the
age of 58, in the early morning hours of April 1, 1994 from
complications due to cancer. We then dedicated NetHack 3.2 in his