It's easy to miss. You have to watch it really close and preferably in slow motion. You blink, you miss it. I didn't mean to imply that he was actually getting "sucked in" like through a portal but rather that he jumped quickly through the flames at the last second just before Peter looked away. Besides, Harry took one of those things point blank to the face and he survived. If Venom were dead, we would have been given a body, not a spin-off movie, which I will link again:
http://movies.ign.com/articles/895/895369p1.html