She was tense, on edge while literally destroying a living being. She tried to keep it at the back of her mind that this wouldn't destroy him completely. That he would still exist, but part of her was disappointed in that. There was a rush at actually killing that she never wanted to give in to. Alex started fighting back and she could pull back, let him finish but it was too late. She wanted this, the Phoenix wanted this and she couldn't pull away.
Even after Apocalypse was gone she wanted to keep going, burn everything, it was hearing Alex that made her jolt and pull back. She couldn't quite get everything back in control and was fighting down the flames herself, stepping back away from them all.