Kirsten Zaat worked for the UN mission in Baghdad in 2003, acting as liaison officer with aid groups operating in the country. She told BBC News about her memories of Margaret Hassan.
Margaret was a human rights defender of unequalled vigour. Her vitality, her verve and her uncompromising stand on protecting the innocent was unparalleled in Iraq.
Mrs Hassan was a 'shining light', Ms Zaat said
Margaret's life was dedicated to lifting up the downtrodden, to holding out the hand of hope to the destitute, to protecting all human beings wherever she found them.
Her humanitarian career spanned three decades and was carried out in far-off destinations, from the Palestinian refugee camps of Lebanon to the slums of Baghdad.
She raged tirelessly against the tyranny of poverty and oppression and fought determinedly to end atrocity and injustice.
Along with two other distinguished humanitarian female role models, Martha Tease and Jill Clark, who were murdered in the bombing of the UN headquarters in Baghdad on 19 August 2003, Margaret's humanitarian assistance - her recent suffering - are now ended.
Those who respected, cherished and admired her continue to help but to also suffer in the knowledge that "we the peoples" have lost another grand woman and an unfaltering champion of the poor.
Margaret's murder constitutes a crime against humanity for which there can be no excuses. The international rule of law exists to protect all civilians, including humanitarian workers.
Around 200 people rallied in Baghdad for Mrs Hassan's release
The Geneva Conventions prevent the taking of hostages. Such war crimes must be tried.
The perpetrators of this murder consider themselves Islamist guardians of the marginalised and the forgotten. Yet they know not the Sharia, which prohibits such brutality.
Murdering Margaret constitutes a serious breach of Islamic law.
It is no coincidence that Margaret's death by a single bullet mirrors the US military's apparent killing of a POW in Falluja.
All parties to this conflict are abusing the laws of war and all parties to this conflict are placing the lives of innocent civilians at risk.
If Margaret were alive today she would be calling upon multinational forces and resistance fighters to respect the rules of engagement in accordance with international humanitarian law.
She would be calling upon the US-led multinational forces and the Iraqi Interim Administration to guarantee humanitarian access in response to aid agencies being denied their right to deliver basic needs and social services.
Her voice is silent now and the bitter irony is that Margaret would have been the first among us to condemn the current attack on Falluja.
Humanitarians have endured too many sad days since the war on Iraq began. The families, friends and colleagues of innocent civilians now lost forever have shed too many tears.
As a tribute to Margaret's heart of gold and her head of steel, to her honesty and to her strength, "we the peoples" must reflect on and attempt to right what has gone so terribly wrong in Iraq. Margaret would permit nothing less.
To all parties to the ongoing conflict in Iraq I say lay down your arms - this is a battle that cannot be won.
Ms Zaat worked for the UN in Iraq and Kuwait
To perpetrators of war crimes and crimes against humanity across the globe I say take heed - it is only a matter of time before the International Criminal Court holds you to account.
To my fellow humanitarians I say continue to speak out loud and clear against atrocities and injustice wherever you find them - the lives of communities in need and your own lives depends on it.
To the Bush, Blair and other coalition governments I say you are perpetuating a counterfeit "clash of civilisations", which has made us all targets. Learn your Crusader and Hezbollah hostage-taking histories well.
To all media outlets, internet service providers and the general public I say stop promoting and viewing hostage video tapes immediately - you are compromising both the lives of captors and your own humanity.
To Margaret's husband and family I say thank you for sharing Margaret and her life's work with us.
Without your support and generosity towards her, and your indefatigable love of her, we would never have known her and she would never have been able to offer so much to humankind.
Margaret, we loved you. Our humanitarian paths crossed in Iraq for a time that was unnecessarily brief and all too weighty. You were a beacon of hope and a shining light to all humanity.
We will remember you as we continue to assist those most in need, to strive to guarantee all peoples their fundamental human rights, to help them to understand their responsibilities to protect their fellow human beings, to uphold the humanitarian imperative and to protect humanitarian space.
Margaret, you died battling for equality and for peace. Rest now in the eternal comfort of knowing that you gave so much and touched so many.
Kirsten Zaat worked at the UN mission in Iraq from May to December 2003 and was in Amman, Jordan, at the time of the bombing of the mission's headquarters in August that year. She was then appointed Head of Office for the United Nations Office for the Co-ordination of Humanitarian Affairs in Kuwait. She is currently based in Melbourne, Australia.