Should I be mad?

I went to the bar with my fiancé, my sister and her two friends. I had to leave to go do something and I told them I was gonna come back. Once I got back, I caught my fiancé smoking a cigarette after I JUST bought him a $90 vape thing. I have been weening him off of chew and cigarettes for the past year (he wanted to quit) and now it's a kinda slap in my face. He said he was gonna tell me but I feel like he was gonna hide it from me if he waited until I left. He looked surprised once I got back because I told them I was gonna be back in an hour but it took me like 30 minutes. Should I be mad and upset or let it go? I feel like he disrespected me by going behind my back like that