The deaths of hundreds of thousands in Syria is not due to a lack of compassion from the international community, but because people are "under the illusion that this is not their struggle," Kayla Mueller once told her parents.

The parents of Christian humanitarian aid worker Kayla, whose death while in ISIS captivity was confirmed in February, shared a letter their daughter wrote to them while serving in Syria.

Her father, Carl Mueller, remembered asking her to come home:

"I was angry. I said, 'This is not your war, these are not your people. You don't need to die for this. Come home,'" he said, before reading the words she wrote back.

"I do believe this is my struggle," Kayla wrote. "Really, in the end, the real reason that 100,000 lives have been lost in Syria is not because people don't care, or don't have sympathy or compassion. But rather it's because people are under the illusion that this is not their struggle, it is not their people and it's not their concern."

Kayla was taken hostage by ISIS in August 2013 after leaving a hospital in Aleppo, Syria. Although ISIS initially clamied that she was killed by a US-led airstrike, three Yazidi girls who were held alongside Mueller and have since escaped told the BBC in September that she was murdered by jihadists.

Speaking at the event on Friday held to honour their daughter at her old university, Northern Arizona University, Marsha Mueller shared the joy her work brought Kayla:

"It was hard not to let Kayla go to all these places she did because it gave her so much joy. The more she helped, the more she got."

When Kayla told her parents she wanted to graduate from NAU in two and a half years instead of four, she justified it simply:

"She said, 'Mom, I've got things I need to do. I need to get out there and do things,'" Marsha said.

The couple also shared how, amidst moments of deep sorrow, they had shared moments of gratitude hearing the impact their daughter had and is having around the world.

A local rabbi once visited them, bringing a message from a rabbi Kayla had befriend in Israel, and just moments later they received a text message from a Palestinian group that their daughter had worked with.

"Kayla was like a voice in the deep forest screaming the truth and screaming reality but nobody was there to listen, nobody could hear," Carl said. "But now she's being heard, she's affecting people all over the world."

Kayla has been awarded multiple honours since her death, one of which was from the Desert Southwest Methodist Conference of the United Methodist Church in June, recognising her and her ministry with the Francis Asbury Award.

"This is in honour, in memory and celebration of what it means to be a disciple and to transform the world and to love others," said Rob Rynders, chair of the regional United Methodist Board of Campus Ministry as he presented the award.