John Gill Commentary Mark 9:26

"And having cried out, and torn him much, he came out: and [the boy] became as one dead; insomuch that the more part said, He is dead." — Mark 9:26 (ASV)

And [the spirit] cried, and rent him sore
We rightly supply, "the spirit", as do the Syriac and Persic versions, "the demon"; for it was he, and not the child, that cried, and made an hideous noise, at his ejection; being filled with wrath and rage, that he must be obliged to quit the possession he had so long held; and therefore, in spite and malice, before it left him, shook and tore him, and threw him into dreadful convulsions:

and came out of him ;
though sore against his will, being obliged to it, by the superior power of Christ:

and he was as one dead :
that is, the child, when the devil had left him, lay as still as if he had no breath, nor life in him:

insomuch that many said, he is dead ;
really dead: that there was no life in him, nor any hopes of his coming to himself again.