Whilst no one would condone the use of his means of removing her if she had just been standing passively, her actions suggest that there may have been a possibility, given the current high terrorist alerts, that she was going to do something more serious.
Nonsense. She was a woman in a sleeveless dress wearing a climate change sash, a member of a group with precisely zero record of violence; she could not have been carrying any weapon because she'd just come through security, and she wasn't carrying anything in which a weapon could have been concealed. No-one else present, including trained security guards, thought she or her colleagues were terrorists or about to do anything serious, otherwise they wouldn't all be calmly sitting at their tables in those clips.
These attempts to justify Field after the event are really all looking a bit desperate.