I saw this one on SciFi Channel many years ago. What I can remember of the film goes something like this: a group of very attractive young women, all blonde and I think part of a sorority, are killing young men. They do so by seducing the young men, and then their torso erupts in an alien tentacled horror which attacks their victim, killing them and maybe consuming them(?). The effects were low-end CGI and kind of on the cheesy side.
There is a male protagonist who eventually does battle with the blondes, defeating them. His girlfriend, a brunette who written as a stereotypical "good girl" type in the movie, was I think at one point trying to join the blondes' sorority or group.
The film ends with a plot twist. After defeating the killer blonde aliens, the protagonist and his super-innocent girlfriend realize they are in love, and decide to consummate the relationship. In the final scene, we see the two in silhouette through a bedroom window, and then we see the brunette's torso erupt in the alien horror, revealing she was one of them all along. It didn't make a lot of sense, but plot wasn't exactly one of the movie's strong points in the first place.
I'm certain I saw this on television. I can't pin down the exact timeframe, but it was sometime between the late 90s to mid-2000's. Given how cheap the F/X were, the lack of any big names, etc. think it very likely it was a made-for-tv movie or a direct-to-video release. It may have actually been a SciFi/SyFy original film.