I don't think he needed any manipulating, he had a gun with him he could have easily shot her if all his intention was to kill her to save the child, his statements as to why he didn't shoot her are that he was afraid the neighbours may hear and he didn't want anyone to get involved (not verbatim). I also thought the answer he gave to if others may have got involved implied he unfortunately would have to kill them, sarcasm by me. He states twice he was concerned others may have become involved and from the way he testified I read it as it wouldn't have gone well for them, one as previously mentioned was if they heard a gunshot and the other when he said he was concerned the neighbours may have heard the commotion when he was killing Christina and she was fighting for her life and he has to leave the crime scene in a hurry.
That small bit of testimony on direct from his own mouth revealed so much about him.
He could still have shot her, but he chose to slash, cut and stab her over and over again. Shooting someone is more likely to kill than stabbing, stabbing takes time, shooting is quick, so if he only wanted to kill her he would have chosen the most effective way IMO
The way he spoke about Grace and Christina revealed much more than he thought it did, amongst his many achievements he was testifying as if he was a psychiatrist and could diagnose them both with the exact same mental health conditions, only someone as special as he could know what deviants they were, despite him never having met Christina, and Grace being a damaged child, I think he seems to have held onto an extraordinary amount of hate and revulsion towards Grace and he was on some way killing Grace as well as Christina, which is why it was up close and personal, he had to make Grace/Christina suffer,