On a show where they couldn't even track the continuity from pink dresses to black ones, it really is too much to expect that anyone on the set in New York should have called the Maine Legislature to ask if they had the death penalty on the books.
Well, believe it or not, the writers did research obscure Maine laws for the witchcraft trials that took place on the show. So, they were motivated when the spirit moved them. Unfortunately, it didn't move them often enough.
And the death penalty issue is but a fraction of the problem. Even the most dimwitted attorney imaginable could have gotten Philip off had his case ever actually gone to trial.
Ah, the exquisite tortures of examining Dark Shadows too closely!!
As for examining this plot too closely, well, back in the Dark Ages of online DS fandom (early 1997) circumstances existed that required that I closely examine even the tiniest minutiae of this storyline. But to reveal the reason behind it would be to reveal too much, as those who know who I am will definitely understand.
You'll simply have to trust that there was a very good reason at the time. It's just too bad that none of us involved in the project thought to save any of it.
So, much like the events of the actual Dark Ages, it's merely the stuff of legend nowadays...