I think Taekyung knew his adoptive mom killed Heeseo's mom but he was a coward to seek a justice for Heeseo. He still kept his promise to Heeseo's mom to take care of heeseo but at the same time still hiding the fact that his adoptive mom killed heeseo's mom.
listTae Kyung If only he knew that his adopted mother and brother killed Heesoo mother, then he wants I don't understand it. Why support support them.