You had a loveless marriage. Actively taking care of your son and your boring husband. You hated it, but, you managed.

While your husband finished some paperwork, you changed your clothes. The way he acted, it made you.. Happy. Yet, suspicious.

'I know nothing about this mans life. He has a partner and a son.'

Your presumed husband thought, This wasn't your husband at all. It was that runaway killer, Yoshikage Kira. Using your husband's identity to get away from his troubles.