He looked away, jaw set. The fact Cat was talking about someone in the past tense, the way she spoke... it only served to imply to Bellamy that his worst fears for Clarke could be a reality. It wasn't as if he didn't know that already and if he were honest with himself, it wasn't as if that was any different than the world he came from. There was always the real possibility of death back home. He thought here, the longer they stayed, maybe they would be able to be safe. With Clarke joining the Resistance, that seemed like an achievement he would have difficulty with. Not unless he joined with her to keep her safe - but then what about his sister? He had to keep her safe as well and he knew the Resistance wouldn't allow him that option. The idea that Clarke could die was one he preferred not to indulge because he didn't like the desperate way it made him feel. It was something he'd already gone through back home and that fact alone made it an easier leap in relation to the subtle path he was being led down.
When she said she thought she was talking to the same people he was, Bellamy's eyebrows lifted and his attention shifted back to her. Cat was right. They did seem to promise more protection, a better life. Something more than just survival. And judging from what Cat described of her life before she got her, it was something she deserved too. It was one of Bellamy's biggest and most easily exploited weaknesses - his intense desire to protect others. It was most dangerous in relation to his people, but that was a circle easily expanded.
"You should trust what you think will keep you safe," he said quietly. "Whatever fight is happening here... it's not ours. You deserve to be safe," Bellamy said. His sister deserved to be safe. And he would do what needed to be done to ensure that. The Resistance would exploit people like Clarke with promises of doing the right thing and stopping evil - but what was right and what was evil? All that mattered was whatever could keep the people you loved safe. "All I know is I need to keep the people I care about safe, and I have always done that no matter what it takes. No matter what I have to do." He knew he didn't know Cat very well, not yet, but he hoped he came off as someone who would do what needed to be done to keep his friends safe.