The Fly (1986)

Seth Brundle, a brilliant but eccentric scientist attempts to woo investigative journalist Veronica Quaife by offering her a scoop on his latest research in the field of matter transportation, which against all the expectations of the scientific establishment have proved successful. Up to a point. Brundle thinks he has ironed out the last problem when he successfully transports a living creature, but when he attempts to teleport himself a fly enters one of the transmission booths, and Brundle finds he is a changed man. This Science-Gone-Mad film is the source of the quotable quote "Be afraid. Be very afraid." Source

IMHO it's the best Cronenberg's movie. The plot is well written, the actors well chosen, the FX amazing, and it has gore, lots of gore.

Seth Brundle: You have to leave now, and never come back here. Have you ever heard of insect politics? Neither have I. Insects... don't have politics. They're very... brutal. No compassion, no compromise. We can't trust the insect. I'd like to become the first... insect politician. Y'see, I'd like to, but... I'm afraid, uh...
Ronnie: I don't know what you're trying to say.
Seth Brundle: I'm saying... I'm saying I - I'm an insect who dreamt he was a man and loved it. But now the dream is over... and the insect is awake.
Ronnie: No. no, Seth...
Seth Brundle: I'm saying... I'll hurt you if you stay.