This is the 8851st most frequent Russian word.
предатель
Traitor.
The word "предатель" is used here as a noun meaning "traitor," directly describing a person who has betrayed others by providing their secrets to an adversary.
Он предатель, потому что выдал наши секреты врагу.
He is a traitor because he revealed our secrets to the enemy.
In this sentence, "предателем" is an instrumental case of the noun "предатель," used in a figurative sense to describe feelings of remorse for not adhering to one's commitments.
Чувствовать себя предателем после того, как нарушил обещание, очень неприятно.
Feeling like a traitor after breaking a promise is very unpleasant.
Here, "предателем" refers to a person being unfaithful to their own principles, again in a metaphorical usage of the term "предатель."