merrin’s death so how??

I dunno – the visuals suggested that the demon cause his heart attack via reaching into his chest and ripping out his heart. Louise Fletcher, while linked into this scene, also suffers a “heart attack”.

The novel was pretty clear that Merrin’s heart gave out thru stress, not thru the demon’s action. In fact, the demon is angry that Merrin dies before the demon can kill him or morally defeat him. EX2/Heretic completely contradicts Blatty’s own scenario – just another instance of how much Boorman despised Blatty’s novel/Friedkin’s film.