The lava dragon blames himself for being unable to save his father, the flame dragon. That does not mean he will allow himself to remain mired in misery. He is now the rightful heir to Gharmund, and he will fulfill his role even if his body, seething with revenge, is permanently frozen solid. However, he must first locate the missing Firedrake.
In the Fight Fire with Fire storyline, the Firedrake is a legendary sword. Its power kept Mt. Vakk dormant; presumably it's absence caused the volcano to erupt.