You have gotta be SWATting me.
"but he's age being under the responsibility threshold should count towards some mitigation"
Why? In a world where 3- and 4-year olds are apparently self-aware and intelligent enough to decide they want gender reassignment surgery, how old do you need to be to realise that sending SWAT teams to somebody's address - especially if you have explicitly told them the person is armed and will resist - is likely to end in a very bad way?
If he's smart enough to work out which Twitter handles are the most valuable for his purposes, and he can work out how to get a group of like-minded thugs to go along with his plan, and he can work out where to get addresses, telephone numbers etc, then how do you figure his age has any relevance on how guilty the little piece of human excrement is? If it had been the first and only thig he did, and it had been to a "random stranger", then maybe - just maybe - he could claim to have not fully understood the potential consequences of his actions. But as it was part of an ongoing campaign - which only ended prematurely as it did because the victim DIED - there is absolutely no room for doubt that Sonderman knew exactly what he was doing. Five years is nowhere near long enough, especially if you remember Herring wasn't his only target.
Still think his age is any sort of defence?