This is why Jews have such a hard time of it. Not believing God capable of such mercies they refuse to be party to them. You said so yourself.
G-d is capable of such mercies. But the question is moot. Marwan Barghouti should not be suffered to live, because if he gets a chance, he will kill again and again and again. Furthermore, he’s poisoning the mind of American youths with the help of his fateful sidekick, his son. It’s primarily Jews he’s gunning for right now, but he’ll do for some Christians, too, if he can, and throw in a few Hindus for good measure. If you have mercy on a rabid dog, and fail to kill him, he will simply bite you. So why bother with mercy on his soul. G-d will take care of that soul whichever way He chooses, if we will only send him up to Him and have done.