First reason, because you're a security risk, as I assume you know. [Tony ticks his reasons off on his fingers.] I solve that problem, means someone won't pull a Zemo on you again later and make the rest of us take you down. [It's not something he'd say to Steve - or maybe it would be, if he was feeling harsh - but he figures Barnes knows the truth just as well as he does.]

Second, because I tried to kill you before, and I admit, that might have been a dick move. [No matter how much he'd thought he deserved it at the time. Tony doesn't want his hands to be stained with more blood. That's not who he is, not anymore. (At least, it's not who he wants to be.)]

Third- [He hesitates for just a moment.] Third, because I think it's what my mom would've wanted me to do. She was a good woman, and I've done a lot to try and honor her memory. If there's one more thing I need to do, then it's this.