This may refer to the halachot of murder:
Bamidbar 33:16-18
Anyone, however, who strikes another with an iron object so that death results is a murderer; the murderer must be put to death. If he struck him with a stone tool that could cause death, and death resulted, he is a murderer; the murderer must be put to death. Similarly, if the object with which he struck him was a wooden tool that could cause death, and death resulted, he is a murderer; the murderer must be put to death.
The Rabbis learn from this that a tool made of stone or wood must be investigated to figure out whether it was likely to cause death in an ordinary case. If not, the killer cannot be executed. In contrast, any blow with an iron tool is always considered serious enough to cause death - even a poke with a needle.
In other words, ANY tool made of iron is liable to be a murder weapon that 'shortens one's days', unlike tools made of other materials. Therefore, as Rashi says, such a tool is not fit to be used to build an alter to Hashem.