I'm surprised they haven't already. They have said things like calling this extremism etc.
Unfortunately, that may backfire worse than they think it will. The moment they identify supporters of what he did as terrorists, then a lot more people suddenly become immune to further propaganda towards terrorism.
The smart thing for them would have been to shut up about it.
I think the title of martyr can be applied to a prisoner as well. Usually a prisoner that isn't allowed to return after his time is served. So even if they don't kill him, the longer he's held, the more of a martyr he is.