I'm not going to lend my calculator to my brother's friend.
Why would you not let him borrow your calculator?
He's a rotten piece of shit and betrayed my trust several times.
You should tell him that treating you badly is bad and it makes him untrustworthy. However, I do not think that retaliating against him by not lending him the calculator is a mistake as we need to strive to make the people around succeed even if they have not done the same thing.
No, I'd rather watch him suffer.
I would suggest that you just give him the calculator and promptly ask for it back as soon as he's finished using it. That way he may realize that you really care for him and probably would also change his ways and treat you better moving forward.