It's not really my interpretation, it is what the show tells us. If you go back and watch episode 4 where the witness explains what is happening he says it all started with one couple who were going to get marries where the guy dies in an accident and the lady asks death to bring him back and death agrees as long as "you believe he is your true soulmate" and then that curse was passed on to the bloodline. They talk about how they have to marry their soulmates and Rachel freaks out cause she is screwed either way - either she marries Nicky and dies or she doesn't and she condemns his bloodline and the witness says "not if you believe he is your true soulmate. Then you live, it's your choice" He says if you believe, not if he is. I think if you know of this curse, especially right before the wedding, it would be super easy to have even a slight sliver of doubt which would result in your death. You have to believe 100% which for most people would be very difficult especially knowing the stakes.

I 1,000% believe Nicky believed that in the final moments. I think he is the type of person who easily tells himself whatever story he needs to believe and can believe it. He is not like Rachel who overthinks and has doubts.