Inital story idea

The idea we have is in the direction of Teen-Thriller.
The story starts with intoxicated teenagers, they are not fully conscious of what they are doing and about to do and are out, just for a good time. One person in the group is done with the cigarette and unknowingly throws the smouldering used cigarette into a window of a house. They walk off to continue with their night. The cigarette that is thrown into the house lights the carpet, and the carpet goes on to light other objects in the house until the whole house goes into flames.
The next morning they find in all the local papers that a mysterious fire has caused a house to be burnt down. They know it was them, however they collectively choose to ignore it and move on. He sustains severe burs to his hands and arms and minor ones to the facial area.
The group are unaware of what they have done. The man of the house also is unaware of who caused the incident and looks into seeking revenge.
Two Months later, someone knows what the teenagers have done and this "someone" is the man who was in the house at the time of the fire, yet he escaped the fire before anyone knew he was in there. The Group decide to take a holiday. A road trip to Cornwall having completely forgotten
what has happened, they begin to receive death threats from owner of the house, however they are unaware of the fact that it is the owner of the house. After a numerous amount of letters being sent to them, a series of events start to take place which are consequences about to be faced due to the house fire they set. As time progresses they try to save each another from the random and anonymous attacks. They start to link up the clues in the letters with the house fire that they caused. A member of the group suggest the idea that the letters and the fact that they burnt down the house are linked together. They start to worry and panic about the situation that they are in. Later that day the man kidnappes one from the gang and uses his phone to message all of the others telling them that they have to meet him urgently in a room. The gang does so and as soon as they open the door they see their friend tied up on a chair getting doused flamable liquide all over him/her.
One of the gang members runs in to save him/her but he gets knocked out by the man. The others decide that they could take him but as soon as they take one step the man pulls out a gun and tells them to freeze, the man then ties up all the remaining teenagers to chairs but realizes that there is a missing chair for the knocked out teen, so he decides to leave him on the floor and pours petrol all over him. He then comes back to the first person he tied up and decides to light each teen individually, he lights up the first one but as soon as he turns around he realizes that the knocked out teen has gotten up but it's too late, before he can react the teen tackles him on the floor and knockes him out with a lamp. He saves everyone in time but one of them is badly injured. The story ends with the medics putting the injured teen in the back of the ambulance and the man getting handcuffed by the police.