" If convicted, al-Timimi faces up to life in prison. "
....is that the best you can do to him?..... I mean do you have "fake encounters" in your country?
Timimi kept his hands free of blood and let underlings do his bidding- that makes it harder to nail him for a capital crime.
You'd think treason charges would be in order but the way the Constitution was set up, trying someone for treason is a very difficult task and few prosecutors will go that route. In our system it's sometimes easier to put an evil man in prison and keep him there if he's charged with some lesser crime than it is to try for the death penalty and risk appeal. for example, Al Capone the gangster was responsible for killing a lot of people but it wasn't a murder charge that ended his career- it was income tax evasion, I believe.
As for Timimi, there may yet be more charges coming if he can be tied to a successful murder conspiracy such as the "beltway shooter" murders.
There is a great web site you might be interested in for following the progress of various terrorist trials. It's called FINDLAW. Another is COURT TV