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.

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. 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.