Does anyone understand the motives behind the ISIS massacre of Shias?
You answered your own question.
The only logical explanation is that they think it will "strike fear into the hearts of the (Shia) infidels"
To ISIS, the Shia are non muslims. ISIS is fighting a war, the quote unquote "real" muslims against the kaffir, and as far as ISIS are concerned, Shia muslims are as kaffir as Christopher Hitchens. Shia aren't even people of the book, just dirty liars, probably hell-sent.