Headcanon for Danganronpa: (includes spoilers) Sayaka and Leon, before the killing game, had a good relationship, either just really close friends or even more, and Sayaka thought Leon was familiar so she thought that if he felt that she was familiar he would be less suspecting of her true intent or inviting him to her room.
In my personal opinion, I feel like Sayaka's actions are somewhat justified since she didn't know everyone would die if they voted Makoto, she just knew that she would get out of the school and see what happened to her group and friends.