He was head of risk management for that division, so gets scapegoated, and she was trying to protect herself. When the size of the problem was understood, she was scapegoated as well. Tuld admitted that when he told her he needed "a head to feed to the board."
Risk managers don't add value to the company.
I hope attack comes in October
-PdxGold hoping for a terrorist attack in the US

I'm not convinced she did. Being one of the few women in a man's world, she is likely to be the scapegoat in situations.like she ultimately was by Jeremy Irons. When in fact, SHE is the one who passed along more than one warning about the problem with the securities and the risk. She even admits to Tucci that maybe she didn't stress enough the urgency. But she didn't have to, did she? It's enough to report to the boss that the risk is tipping such that it could bring down the company.
But if she did get Tucci fired, it's probably just a matter of she had to provide a list from that dept. He was older, not quite as sharp as Peter, he got paid a lot moreso it was him. Nothing personal.
But as I said, I'm not convinced she did do that. She says she didn't. It's only Will's look in his eyes, when Tucci asks if it was her, that made Tucci think it was her. Will, as we know, is an automaton.not quite human. He doesn't walk around giving out information and help. He's unabashedly in it for the money and walks hand in hand with Kevin Spacey's character. Until the end, when he tips off the young guy, when asked, that he is indeed going to be fired that day. But he doesn't just give a look. He tells him outright, "Yes."

Also like he said she's a beep ?

Why do execs fire people? So they can make more money and look good.