What they are about to do is just tit for tat. They have been attacked by dragons recently, so it is just right that they attack the dragons too. Their home plane has been destroyed, so the dragon plane should be destroyed too.
They will not leave until the dragon plane has been destroyed. They are determined to do that, and nothing will stop them. Not even death. They are immortal. They will simply return after death to make sure their revenge is settled.
Instead of fear, most of the Supreme Beasts felt excitement. Even the ancient ones that should have a good handle on their emotions felt emotional. This is because this fight is a precious opportunity for revenge that they never thought they would get.