I don't think the law has caught up with the implications of social media - in some ways, it hasn't caught up with the motor car, but that's another topic.
I don't know what the charge will be - but if I were to try and express it, it would be something like 'offending standards of common decency/humanity'. You don't wish a cruel death on someone just because you don't agree with them.
It was like when that American academic (what is it with university staff - do they think they're on a higher level and can say just about anything??) who wished the late Queen had died a painful death because of the British Empire??? These people are crazy and have lost sight of decent behaviour. If you don't like the British Empire, no problem, but why should that translate into wishing a painful death on an old woman?
I don't know if people like this have always been amongst us, or if social media and other factors have emboldened them and their repulsive views.