While it was true that his breakthrough into the soldier stage activated several abilities of his bloodline, Seth had no idea which abilities he had gained given that what he possessed was a merged bloodline, and even though from his memories, he knew that vampires could feed on blood to rapidly heal their injuries, and even replenish their qi depending on the vampire's cultivation level, he had no idea if it would work for him which was why he had been hesitant to immediately kill Amelia.
After seeing Diana brutally kill her husband, Seth had hoped she would be grateful to him and let bygones be bygones, but unfortunately for him, the ungrateful woman had instead decided to attack him, forcing him to feed on Amelia and hope her blood could help him heal his injuries, and in the end, he wasn't even allowed to properly feed as Diana attacked him.
