When Ms Datcu fell pregnant, cannabis smoker Das initially told her he did not want children because he was not a strong person.
After Das failed to turn up for work on March 14, he told his manager that his partner was being unfaithful.
He went to social services and said to the manager he loved Gabriel “too much, too much”.
Ms Johnson said Das’s suspicions of infidelity were “completely unfounded” and caused “significant difficulties between the couple”.
Das left his mobile phone in their bedroom to record what she was doing while he was out, Ms Johnson said.
“His paranoia resulted in him repeatedly calling her from work to check up on her, and then to leaving work during his night shifts to come home to see what Ms Datcu was doing and to search for men he believed were hiding in the flat,” the lawyer said.
Das had assaulted his partner at least three times as he accused her of being unfaithful, the court heard.
Ms Johnson said: “Things got so bad that on the occasions when they spent the night together he removed his shoe laces and tied Ms Datcu’s underwear to his own, to ensure that she could not get up without him knowing.”
Ms Datcu was “concerned” about his behaviour but stood by him for the sake of their child.
But she was “shocked and upset” to get a call from social services about Das’s allegation that she was an “unfit mother”, days before the killing, Ms Johnson said.