Siegmeyer went hollow because he got the hydra down to one head, and his already-cracked mind snapped trying to comprehend what started happening then.
Siegmeyer went hollow because he got the hydra down to one head, and his already-cracked mind snapped trying to comprehend what started happening then.
No, he went Hollow because of the player. Siegmeyer prided himself on his combat prowess and adventuring skills, and was in Lordan looking for some adventure. But when you, the player, constantly rescue him from situations he has little hope of getting through, it makes him look bad. You destroy his confidence in himself, and when Sieglinde delivers her mother's last words it was likely the final straw for poor Onionbro's confidence.
It's likely that in a last desperate attempt to regain some self confidence and save himself from becoming Hollow he attacked the hydra in Ash Lake, but failed, and upon going back to the bonfire became such.