The big question (spoilers)
Serious spoilers, do not read on unless you've seen the film, you've been warned.
The question is this:
Did Cassie go to the bachelor party planning on giving her life, thus ensuring the destruction of Al, or did she just see her death as a strong possibility and was prepared for either outcome?
I think the tally marks onscreen suggest she planned herself as number 4. I suppose if she had lived, maybe there wouldn't be a fifth tally, or perhaps there still would. Nothing says she was counting to any specific number.
I also feel like her overwhelming survivor's guilt that she "wasn't there" with Nina suggests she wanted to sacrifice herself. This mission was all she had left.
Also, being as smart as she is, she would know that if anyone could weasel their way out of the fallout from the video, it would be these guys. It's my opinion that she knew the only way to be absolutely certain they would go down is for murder.