I think it all would return to the fact that Thien cares about Muey, in his own terms. Remember when Thien asked Muey to quit her job when they found out they'd be working in the same company? When they had that confrontation, Thien was saying that he wanted her to quit so that by the time they get divorced, nobody would know that Muey is married in the first place and she can move on with her life.
There's that episode as well when Thien buried her with work, so that she'd have no time to entertain her feelings towards Thien. This is right after he found her diary and knew that she's in love with him.
When he said those mean things about not going to fall in love with her, i do believe that he is trying to get her to stop having hope of being in an actual relationship with him. Remember that this is after he heard Muey told Yada that she has been in love with him for years.
He really cares about her, but he thinks he's incapable of reciprocating her feelings (which i believe got something to do with him never actually falling in love before in his life)
So when the whole hacking plot got revealed he feel cheated and fooled. He really intended to protect her, hence why he wanted to meet up Chayt and Khun Baramee. I don't agree with him locking her up but i guess that's his method to make sure Muey wouldn't go around to hack anymore.
Lengthy post phew, look at what Thienwat that smartass made me do
