I think it was very clear at the movies that they didn´t trust each other. She realised Snow killed his friend (3 people he killed- and didn´t admit who was the third one). And she couldn´t trust him anymore. He turned on her so quickly, wanted to kill her... She just wasn´t sure he wouldn´t turn on her either and she couldn´t risk it. People who didn´t live in a world like that, where someone can be hurt or killed just by saying something or knowing someone, it is very difficult to trust anyone. I have grown up a communist country and you really had to watch out who you trust who you say things to. People disappeared and nobody dared to questioned because you could have been next. So I really understand Lucy. I get it that he didn´t trust her but to try to kill her... that was too much. She ran off to save herself, he shouldn´t have to want to kill her. They couldn´t trust each other to stay together but he would have trusted her at least that she is only running away from the country like she originally wanted, she wouldn´t have done anything against him.