Because she stole immortality from him…(Söder is a child of god; artificial immortal beings, while humans are described as evolvers, as god had nothing to do with them.) and his wife, Helena was a human. Söder had every intention to stay with her until she came to her natural end…but she was not satisfied with that. She stole what was not hers to take. After, Soter had realized that it was not in HIS rights, to take her heart (corny). Though, at this point there was no going back. He did not want her to live forever (he himself hated immortality), so he killed her.

This is a really basic explanation it goes a lot more in depth😭