That's a very good observation! But you are remembering the movie not the book.
Devil's Snare kills its victims by constriction and reacts poorly to fire.
In the film version Hermione recites a poem about Devil's Snare:
Devil's Snare, Devil's Snare
It's deadly fun
But will sulk in the sun.
She and Harry relax and it lets them pass through, but she has to send sunlight at it for it to release Ron.
In the book it has appeared twice in the series.
First, in SS, it was Professor Sprout's protection against the theft of the Soccer's/Philosopher's Stone. It proved an obstacle to Harry, Ron and Hermione and Hermione defeats it by setting it on fire.
Second, in OoTP, potted Devil's Snare is sent to Unspeakable Broderick Bode while he is in St. Mungo's Hospital for Magical Maladies and Injuries (possibly by Lucius Malfoy, but almost certainly by somebody working for Lord Voldemort) . Healer Strout, who is in charge of Bode, mistakes the deadly plant for a Flitterbloom and it later strangles the comatose Bode.
So if the plant can really only be killed by fire it makes sense that the plant would be able to kill Bode and the lights in the hospital would have no effect on it.
