SOLVED. Fiction, a woman kills workers in an office space that are trapped in the building. [s]

[DSouza]

Here are the details I remenber about the book:

In an work environment (most likely office space) the boss and 5 or 6 more workers, (maybe it was not a regular day and it was the last day of the boss in this work, I'm not sure) discover that they are trapped in their floor because the elevator was sabotaged. They discover later, but we know almost from the beginning that a woman is trying to kill them all. Meanwhile we have a flashback story about the guard of the building. I´m not sure if he is all the time in the building whitout knowing what is happening, but is probably that.
I read this book in 2014, but I dont know the year of the book.

[Kris]

DSouza, can you tell us more about the female killer who is revealed near the beginning - her job, motive, weapons, skills, etc.?

Spoilers are okay - just give us a clear warning before revealing a spoiler (although you mentioned the killer is revealed very early on). Perhaps her motives are a spoiler?

[Cupcakedoll]

[DSouza]

Yes, her motives are spoilers.
And the book is Severance Package, thank you very much Cupcakedoll.