St John doesn't just "mention war between the devil and Christians".
He is specifically speaking about enmity between Satan and "THE WOMAN" from Genesis 3 which he quotes verbatim.
You made a generalization about the Devil not Mary so the emphasis or presence of Mary does not undo the fallacious logic:
"St John mentions a war between the devil and Mary + saints, therefore the devil is only seeking to harm Mary + saints."
This is the fallacy of hasty generalization. Just because St John mentions a war between the devil and Mary + saints does not mean that the devil is also not leading astray, further perverting and destroying mankind as a whole too.