Have a thought ever comes to mind that if you water a Magma Slag, it would become a Dust Zombie or if you light a Frozen Shambler it should also become a Dust Zombie? I personally wonder why curse zombies also do fire damage when attacking. Is this how Spiral Knights logic works?
If you have any wonders on an other npc, feel free to wonder. (Don't ask about jellies doing piercing damage. Jellyfishes sting like heck)
Curse zombies deal fire because they were first created to go with the Darkfire Storm Citadel, which was a mix of curse and fire.
Also, while it may not be what you're looking for, all zombies but slags are weak to fire. They take much more fire damage than other monsters and fire usually lasts longer against them.