During the final when Dave was shown anger at Sky for using him in the game and he wanted to kill her during the last part of the challenge, but it seems he was also trying to kill Shawn along side her like he wanted them both dead! since at the end of the episode when Dave found out Shawn was not dead he screamed "NOO!" 

So i can only think of three things on why Dave was also angry at Shawn 

1. He was feeling jealous of Shawn since he got the girl he wanted and didn't get his heart broken

2. Shawn said something to Dave during the episode that hurt dave's feelings. 

3. When Chris shown the tapes of Shawn talking badly about Jasmine in the confessional Dave grown to dislike him since he felt Jasmine was being used by Shawn and that reminded him of his own relationship with Sky. 

4. It is a mix of all of this.