That was never really explained. I assumed Jin Tae might be taking advantage of Do Joon somehow but it seems he really cared about him.
If my thought-dreams could be seen, they'd probably put my head in a guillotine.
He probably sympathized and kind of saw him as his younger brother.
shareI thought it was quite touching, this hard ass good looking guy being friends with do joon, and not only that going out of his way to make sure do joon stood up for himself and did take any crap. I suppose from a story point of view it made it easy for us to believe jin tae really was the real murderer and used do joon as a scrapegoat, like he did with the car wing mirror, but no he really did care for him and even went to pick him up from jail, when even his mother didnt greet him.
shareIt's both things, he helps him and at the same time exploits him every now and then.
shareJin Tae clearly shows a tendency to control and dominate others. You can see it when he beats the school guys up and really enjoys it. So, hanging out with Do Joon somehow makes Jin Tae feel superior because he has someone to easily manipulate, and at the same time, still feels safe (he blames his fault on Do Joon).
Otherwise, I think he does care for Do Joon a little bit, but not in a sincere way.
Do Joon and his Mother seemed to be easy targets by those stronger than themselves. I would agree that Jin Tae made himself feel superior around Do Joon and as we see later, he manipulated Do Joon's Mother into large payments to free Do Joon.
"I'd rather be hated for who I am, than loved for who I am not".