Hi Sug,
He may not be entirely happy in his current relationship with his girlfriend, but may have reached a point where he just feels that it is safer to stay with her and not rock the boat.
Both he and his girlfriend may have lost the initial spark for one another that they once had. They may have basically settled into a monotonous routine with their relationship that has left him feeling very uninspired and bored.
Whenever he is around you at work, he most likely feels the spark that he craves and so deeply desires.
The way the both of you talk and stare at each other probably makes him feel a lot more alive than he normally is whenever he is with his girlfriend.
He enjoys talking to you and often doesn’t want those moments to end. That is why he was glaring at your friend when she took you away from him during one of your conversations.
So yes, he is most likely interested in you. However, his situation is complicated.
He is tempted to take his interaction with you further but he is holding himself back. A big part of that is the fear of losing a relationship that is already established with his girlfriend.
But also, he may be fearful about work relationships. He has told you about how the work relationship of a friend of his died.
Hence, he may be fearful that if he were to take the risk of leaving his girlfriend in order to pursue a relationship with you, he may end up regretting it if the relationship were to end badly.
So basically, he may be unsure of what to do at this point. Until he figures that out, he may try to keep talking to you whenever he can in the hopes that he can learn more about you.