JUDY WOODRUFF: Earlier today, friends and family of the Islamic State’s latest Western beheading victim, aid worker Peter Kassig, said goodbye to the 26-year-old.

Chief foreign affairs correspondent Margaret Warner reports.

MARGARET WARNER: Kassig, who converted to Islam after his capture in 2013 and took the name Abdul-Rahman, was memorialized this afternoon at an Indiana mosque.

Among the speakers, prominent Syrian Sunni cleric Sheik Muhammad al-Yaqoubi. Al-Yaqoubi was among the first Syrian clerics to call on President Bashar al-Assad to step down in 2011 after government forces cracked down on peaceful protesters. He was forced into exile later that year.

But he’s also a vocal critic of the Islamic State group. Two months ago, he released an open letter to its leader, Abu Bakr al-Baghdadi, telling him: “You have misinterpreted Islam into a religion of harshness, brutality, torture and murder,” which he called a great wrong and an offense to Islam.

I spoke with Sheik al-Yaqoubi yesterday.

Sheik al-Yaqoubi, thank you for joining us.

SHEIK MUHAMMAD AL-YAQOUBI: Thank you.

MARGARET WARNER: Why did you agree to speak at Peter Kassig’s funeral when the family asked you to?

SHEIK MUHAMMAD AL-YAQOUBI: Well, Peter Kassig, Abdul-Rahman, sacrificed his life for the sake of the Syrian people.

He went on a humanitarian mission as an aid worker to help save humanity, to show sympathy to the Syrian people, solidarity of the American people with the Syrian people. So it’s our duty as Syrians to stand by his family, and to stand by his community, and to stand by the American people who gave this example of bravery in this difficult time, when ISIS is slaughtering everyone.

MARGARET WARNER: Why do you think the Islamic State group is staging these violent beheadings of Westerners, even aid workers like Peter Kassig?

SHEIK MUHAMMAD AL-YAQOUBI: They carry hatred to the world, to Muslims and non-Muslims alike.

You see he converted to Islam. It didn’t help him. He was an aid worker. It didn’t help him. What kind of heart kills someone who came to help the people of Syria? What kind of a man even in Islam, between rockets, kills his Muslim brother?

MARGARET WARNER: So, why are they so successful, apparently, in attracting recruits from not only all over the Islamic world, but even Western Europe and here in the United States?

SHEIK MUHAMMAD AL-YAQOUBI: It’s the Assad regime and the atrocities of the Assad regime, which still continue.

It’s a killing machine; 200,000 people have been killed over 3.5 years now. So, as long as the atrocities continue, you will find groups like ISIS succeeding in recruiting more people.

MARGARET WARNER: Now, of course, you were an early opponent of the Assad regime. There were peaceful demonstrations then.

Why is the anti-Assad movement really now apparently being led by the most extreme elements in the Muslim community?

SHEIK MUHAMMAD AL-YAQOUBI: Because the voice of the people was lost within of all of the fighting.

The Assad regime is very cunning. It opted for violence and extreme use of power from the very early days of the uprising. And it released from prison the most extremist Islamists, knowing that they will opt for carrying guns and fighting and revenge. So that is how the shape of the Syrian uprising changed from the beginning.

MARGARET WARNER: You said earlier this week that al-Baghdadi, the self-proclaimed ruler of this Islamic State caliphate, was going to hell. What do you mean?

SHEIK MUHAMMAD AL-YAQOUBI: He’s against Islam. He’s non-Muslim, according to the Muslim standards, because he’s allowing people to kill Muslims, referring to the Book of Allah, wrongly using religious texts.

This is anti-Islamic. He’s going against God. He’s going against the message of Islam, Mohammed, peace be upon him. If he repents and come in a court and defend himself, he won’t have any one single verse of the Koran to defend his opinion in killing innocent people.

MARGARET WARNER: What will this hell look like?

SHEIK MUHAMMAD AL-YAQOUBI: For him?

MARGARET WARNER: Yes.

SHEIK MUHAMMAD AL-YAQOUBI: Hell will — for him, God knows what type of punishment he’s going to receive for this savagery which has never been witnessed in modern history.

You know, in Islam, we have never seen any group as extremist as this group. This is the most dangerous and serious group that existed ever in the history of Islam. It constitutes a threat, not only to the Syrians or the region, but to the whole world. Muslims and Islam carried mercy to the world. And this is totally against the very nature of the message of Islam.

MARGARET WARNER: What would you say to young recruits and would-be recruits? Are they headed for the same fate?

SHEIK MUHAMMAD AL-YAQOUBI: Well, first of all, they shouldn’t be lured by any propaganda that the Islamic State is waging through the Internet especially.

So joining al-Baghdadi is an act of sin, is an enormous sin, is an act of crime. It’s a crime against humanity. You are distorting the image of the very religion and the very God you are worshiping.

MARGARET WARNER: Now, you got more than 100 Islamic scholars of some prominence to sign this letter, but do you think there has been enough significant opposition and outcry from the moderate Muslim community, the nonviolent Muslim community in general against the Islamic State group?

SHEIK MUHAMMAD AL-YAQOUBI: You’re right, Margaret, to put this question. There has not been enough, indeed. And we regret this.

We think that mainstream media in the Arab world has not been highlighting this important issue by bringing religious scholars on prime time to address our Muslim fellows across the Arab world and the Islamic world.

MARGARET WARNER: But, I mean, these are your communities. Can not the communities themselves generate this kind of counterforce in all these other countries where some of these recruits are coming from?

SHEIK MUHAMMAD AL-YAQOUBI: It is very clear that it is incumbent upon all Muslims to inform if they know of anyone traveling, because they’re doing service to God by informing of anyone who is joining this gang.

This is a group of gangsters who are distorting history, distorting the history of humanity even, not only the history of Islam.

MARGARET WARNER: Now, three years ago, when you first came out against Assad, you talked about wanting to build a tolerant, democratic Syria, where all faiths, Sunni and Shia, other faiths could live side by side. Is that dream over?

SHEIK MUHAMMAD AL-YAQOUBI: No, it is not over.

The moment Assad is toppled, you will see that 70 percent of those people who are fighting now will lay down their weapons and they will go back to their homes to reunite with their families and rebuild the country.

Syrians are — they presented a beautiful example over centuries of harmonious coexistence between all the groups. Damascus is the oldest city that has been inhabited in the world for 10,000 years continually. And they are capable of doing this again.

MARGARET WARNER: And you think, after all this violence, all this savagery, it can be put back together?

SHEIK MUHAMMAD AL-YAQOUBI: Syria can be put back together. Its long history will not be erased by someone like al-Baghdadi or a group like ISIS.

Syrians have a lot of hopes. And they seek the help of the friends of Syria and the world to get rid of both the Assad and ISIS.

MARGARET WARNER: Sheik al-Yaqoubi, thank you so much.

SHEIK MUHAMMAD AL-YAQOUBI: Thank you.