I believe Vera hanged herself because she was becoming more and more unhinged.
1 the terrors of the 48 hours. Seeing one person die is bad enough but 8 and then you'd killed one yourself. Look at the effect that would have.
2 After she killed Lombard she saw the figures had been removed. So now she's killed the only person she had a chance of survival with.
3 so, there was still a killer on the loose. therefore he'd killed 8 , her one so her chance of survival was slim and then she had to get off the island.
4 if she survived all of that, she was never ever going to be rid of the guilt of what she did to Cyril. She was never going to be allowed to forget hugo's last words to her.
5 with all of that going on in her head , the fact that the killer was on the loose which she knew when she entered her room and saw the noose, she saw her opportunity to escape the torture of her mind and took it.