In the books, the curse that Molly Weasley used to kill Bellatrix is described as making Bellatrix freeze, and then for the "tiniest moment of time", Bellatrix knew what happened, before toppling over dead.
The main spell that we know that can cause a person to freeze is "Petrificus Totalus". However, that spell is not fatal. My theory is that Molly Weasley used an extra-powerful version of Petrificus Totalus that paralyzed literally everything: including her heart and lungs. This explains why Bellatrix was alive for a split second after, before dropping dead because her heart had stopped.
