Shen can be thought of as a soul, or something that makes a creature sentient. In the conventional view, all creatures commonly called humanoid have shen, but common animals do not, though there are those who contend that they do, of course.
It is generally accepted that most undead no longer posess shen, though exceptions have been documented.
In all cultures, killing a creature with shen is a serious matter. Whether these matters are dealt with by vendetta, blood feud, weir-geld, or a complex law enforcement mechanism, killing a creature with shen is seen as a weighty matter. Unfortunately, this does not always prevent wars. In point of fact, it can often fuel them.
There is no broadly accepted test for the presence of shen, so it can be a matter of some controversy.