This may refer to the halachot of murder:
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. So if someone threw a pebble, it took out someone's eye, and the victim got infected and died, the pebble-thrower would not be executed.
In contrast, any blow with an iron tool is always considered legally serious enough to cause death. If someone died from a poke with a needle (however unlikely that was to happen), it is considered murder (Mishneh Torah Hilchot Rotzchim 3:4).
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 altar to Hashem.