Should Islamic leaders who privately excuse and endorse violent behavior be treated as criminals?

Yes. They are part of the problem. Islamic leaders who support violence are also the same ones brainwashing people to commit it in the first place. If they aren't removed from their leadership positions and replaced with reasonable people who will take a stand against Islamic terrorism then how are we going to solve the problem? Any clerics, like imams, who support Islamic violence should be called out, opposed, and removed. We must stop sending aid and keeping relations with Islamic governments that finances terrorism (such as the Saudi and Qatari gov'ts). And we should stop working with Islamic advocacy groups that support terrorism (such as CAIR when they financed Hamas). We're at war against Islamic terrorism and we must treat ALL who support it as criminals. Don't let the traitors and cowards tell you otherwise.

What leaders are you talking about? If you mean in the US, there are measures that can be taken against anyone advocating violence, especially towards the government. If you mean in other countries, we already do. That's why we've been bombing ISIS for a year. Many leaders have been killed, including Jihadi John. I don't know how we could treat them any more like criminals than by killing them.

Should people who privately excuse and endorse violent behavior toward abortion clinics and doctors be treated as criminals?

No. Hate speech is still free speech. Just cause I don't like what they're saying, doesn't make it criminal. That would lead to all sorts of problems.

Yes, absolutely

If something is done privately, how would anyone know?

Your are getting dangerously to prosecuting people for their thoughts.

No, because killing world leaders is strongly against international law.

Yes and every Mosque in the USA closed.

God, apply that test to Christians and half of Congress and most of Fox News would be in jail.

Yes, but how are we to deal with them?