Ok so why the dad is saying sorry to them did author anywhere said he is responsible for ...

Ok so why the dad is saying sorry to them did author anywhere said he is responsible for taejun death?? Like the dad literally left his wife and son for taejun hyung so what really happened between them ? I didn't understand why they all are angry at him when taejun is the one who has feelings for the old guy right!!! Gosh idk anymore