Did I cross the line?

Well me and my husband are looking after my brother in law because his parents are on holiday in spain. My brother in law is 14 and he has been smoking since he was 9 and I wasn't really happy about it at first but now I realise it's none of my business. I took away his lighter and his cigarettes so he couldn't smoke as I wasn't happy about it at the time. He had some cigarettes left but he didn't have a lighter so he came up to me and said "hey can I please have a lighter?" so I said "sorry hun no you are to young to smoke" he then said "I'm gonna be very angry if I can't have a smoke" so I said "well I guess your gonna have to be angry aren't you" he then lit his cigarette with the toaster so I said "you little **** I told you not to smoke your only a little boy" he then said "desperate times call for desperate measures" so I said "well I couldn't give two flying ***** if your parents let you smoke but your not smoking here" he then called me a ***** so I slapped him across the face. He then ran upstairs to his room crying and I now feel very guilty. Should I give him back his cigarettes and lighter and apologise to him? Plus I don't want him hating me either because I love him as a family member with all my heart.