Wesley Williams killed Yvonne Walsh and her seven-month-old son out of anger after she ended their relationship as he planned to propose to her.
Police found that Williams made comments on Facebook almost immediately after the murders.
In a post to his own Facebook account timed at 8.54pm on Friday May 31, Williams wrote: "Sometimes we just have to do things we shouldn't."
Inquiries by police also proved that Williams told another social media user he was in good spirits, and had then posted "I am sorry it had to come to this" on Ms Walsh's Facebook page.
The 29-year-old cannabis user was also captured on CCTV doing "high-fives" with a friend in a fast-food outlet in the hours before he was arrested by police.
Friends of Williams also told police he had seemed fine after May 31 and was "joking, laughing and smiling" as he played pool on a computer.