Iceman cannot be killed while in his Ice form. Thats been a general rule for a while. Since emma frost took over his body, and he finally learned a thing or two about his own powers.
Then came X-men Forever in which he learned how to spread his consciousness around, and has done so a few times (turning into water, water vapor, mist.)
Then he once got shot by an arrow and blew up. His head was the only part left intact. What he did to get his body back? Stole the water of a "demon" killing her while reforming himself
Hell even in human form he cannot be killed. Northstar blew him up and he came back as mist 😐