When Christine turned the scorpion, that signalled that she agreed to marry Erik, and in return he flooded the gunpowder so that the explosion would not be set off. Erik believed that she would go through with the marriage ceremony, then kill herself, and he accepted that bargain.
But turning the scorpion still left the two men drowning in the torture chamber. Christine had to make a further bargain to save their lives. This is how Erik described the scene to the Daroga afterwards (Ribiere translation):
'But as you were both screaming for your lives because of the rising water, Christine came to me. She looked at me with those beautiful blue eyes, and swore to me, as God was her witness, that we would live as man and wife! Until then, in the depths of those eyes, all I had seen was the promise of death; it was the first time I saw the promise of life. She was sincere. She would not kill herself.'
After rescuing the two men from the flood, Erik took the Daroga home, and locked up Raoul in the dungeon as a hostage. Then he went back to Christine, and that was when he kissed her forehead, and she appeared to accept the kiss. I think that not until then did he really believe that she would live with him. It was not so much that he thought his kiss would kill her, but that she would not want to live. He could of course prevent her physically from committing suicide – he had done so before – but I think he believed she could will herself to die, if forced into marriage with him.
As for the forehead kiss, I think that was a measure of Erik's timidity in a totally unforeseen situation. I don't think he had ever imagined himself kissing her on the lips – not while she lived, anyway.