Splinter
(2008)

Seth (Paulo Costanzo) and Polly (Jill Wagner) go camping to celebrate their anniversary. While camping they are carjacked/kidnapped by an escaped convict Dennis (Shea Whigham) and his girlfriend Lacey (Rachel Kerbs).

Through a series of minor mishaps, hitting a dog, blowing a tire, the foursome ends up at a gas station.

Lacey is first to be killed by a monster and because she was infected her body begins to reanimate. The gas station owner is dead, so Dennis (the convict) and his two hostages lock themselves in the tiny gas station to get away from the thing that killed Lacey.

The three of them begin to bond when Lacey (monster) starts banging her dead head against the glass gas station windows like a zombie.

The convict relizes he was infected by a splinter he had in his finger ealier.  The three are trapped in the gas station and to prevent the infection from speading futher  Seth cuts Dennis's arm off with a box cutter and breaks the bone with a cinder block.

Dennis admits that he had shot a man while trying to steal his truck  and the man's spine had shattered, his wife went broke caring for him and paying medical bills and then the man died a year later. Dennis said that he had planned to escape from prision and give his victim's widow his "nest egg" savings because if he was getting a second chance so should his victim's widow.

A cop comes by and is killed by the monster (Lacey). The trio figuires out that the monster follows body heat and thats how it chases them so they lower seth's body temp, he runs for the car but the car has no keys. Set takes the cops gun and starts shooting the monster.

Dennis lures the monster inside so the other two can get away. He is bitten by the monter on the neck and shoulder. Dennis jokes to Seth that they should cut his head off like they did his arm, he then asks Seth and Polly to take his key and give his money to his victims widow. They agree and start running away from the gas station, Dennis uses the shot gun to shoot the propane tanks blowing up himself and the monster.

While Seth and Polly walk safely into the sunrise, you see the dog they hit with the car the night before, he has splinters sticking out of his mouth that start to twitch implying the monster will live on.
 

Thanks Melissa S!