Fitch seems to have a genuine sadness when he is about to gas Sil in the beginning and Sil is shown to have a deeper connection to him in later scenes like when she controls his speech. Is Fitch the biological father of Sil, like using his sperm to fertilize the egg that creates Sil?
It's unknown whose sperm is being used by him to fertilize the egg responsible for her creation but unless it's that of Fitch himself, I don't think that amounts to him being her father except in terms of indirectness. However, I think his sadness comes more from having to kill a being little more than a child and also because he had never performed an act related to fatality, let alone (the real possibility) being sadness in aborting a dream that he could have achieved which was combining an alien and human together