I think that Sathit is/was a rational person but his love for Jee has made him bitter. Deep down he knows he loves her and wants to protect her but seeing a video of the girl he became to love, trust and want to protect had ran over his girlfriend then how is one to feel after seeing that?
He prayed that Jee wouldn't turn up to his house so he can give her another chance and believe her actions can be justified. But when she turned up, it just proves to him that it was all lies and what Sitta said was true. The girl who was standing in front of him is a murderer, a girl who will trade her body for her beneficial uses, a girl who had lied to him. He felt stupid, blind and betrayed. Jee had given another chance of LOVE. A LOVE that was given to him by her kindness, her sacrifice and her protection.
After Sathit had slept Jee, he tries to make himself believe what he did was right and it was revenge for Tiw. He started to play the mean and hurtful Sathit so he doesn't become a fool to trust Jee again. But his heart can't get rid of her.
He had always trusted Jee and Chaiyan's friendship but now he can't trust her, when Piak's dad asked him to talk to Chaiyan and questioned Chaiyan and Jee's relationship to Sathit, it got him all fired up (he immediately stood up when that question popped up). No matter how much he believes he dislikes Jee now, his heart can't lie. Jee was his and he doesn't want Chaiyan to have her so he goes to stop her before anything happens and using Piak as an excuse.
The jealousy is there, his actions and words says it all. He wouldn't of try to drive her away if he wasn't and he kept trying to kiss her. Before the "love making" night he was always a gentleman. When he used to accidentally touched her (hand, arms, waist) he would always apologize but now he grabs her to hold, touch and even more KISS.
His heart is breaking and suffering, I think that's why he went to her condo. That caring and loving nature of Sathit is still there for Jee. The pain on his head (good work Jee) was nothing compared to his broken heart. I think he wanted to rekindle their love but was interrupted.