Yes, he can be killed. It simply requires a significantly greater power than it did previously. As an Omega-Level mutant, his abilities have expanded to the point he is very difficult to harm and even harder to kill.
- Iceman in the canon Marvel comics Earth-616 has experienced "secondary mutation" which has increased his powers exponentially. After an interaction with Emma Frost who temporarily took over Iceman's body and powers, his mutant abilities exploded with Iceman becoming an Omega-Level mutant.
He retained all of his previous abilities increasing them to levels previously unreached. He can create as much ice as he wants, supercooling materials within a few degrees of absolute zero, even controlling molecular action in materials normally resistant to such control such as Sentinels.
Not only does he possess the power to extract heat from materials, he can also instantly freeze anything with water in it, even the cells of a living creature instantly to ice.
In the past, he covered himself with an icy sheath which offered him protection against injury. In his current incarnations, he has become capable of becoming ice and even if he is harmed or even severely injured, as long as he stays in his energized ice form, he can be "reconstituted" and restored to his normal body.
In an injury which should have proven mortal, fighting against the mutant Black Tom Cassidy, Iceman has a hole blown through his chest. When he turned back to flesh, the hole was filled with ice and slowly turned back into flesh.
In the 2001 X-Men Forever series, Fabian Nicieza wrote that Iceman had incredible power he didn’t know how to use, and in the final issues, Iceman is melted by Sentinels and essentially evaporated. Yet he reforms himself, which has to have involved cellular or molecular manipulation, before he freezes an entire spaceship.
Iceman is essentially a mind capable of converting itself into ice and injuries sustained when he is a creature of ice are temporary unless he is atomized completely.