**Because he sold Gendry out to Melisandre.**

Note that he's not on her kill list in the books, only in the TV show.

The full list from the show is as follows (more or less copied from [here](http://www.tvguide.com/news/game-of-thrones-arya-kill-list/); possible spoilers):

> - **The Hound**  
>   *Why:* For killing Mycah, the butcher's boy.  
>   *Status:* **Alive.** Although Arya likely believes otherwise after she left him for dead following his battle with Brienne of Tarth.

> - **Meryn Trant**  
>   *Why:* For (presumably) killing Syrio Forel.  
>   *Status:* **Dead.** Arya slit his throat in a Braavosi brothel where Meryn was abusing underage girls.

> - **Cersei Lannister**  
>   *Why:* For her role in Ned Stark's death.  
>   *Status:* **Alive** - for now.

> - **Joffrey**  
>   *Why:* For ordering Ned Stark's execution.  
>   *Status:* **Dead.** Joffrey was poisoned by Olenna Tyrell and Petyr Baelish at his wedding to Margaery. Unfortunately, Arya's sister Sansa was suspected of the murder and had to go on the run.

> - **Ilyn Payne**  
>   *Why:* For executing Ned Stark.  
>   *Status:* **Alive.**

> - **Polliver**  
>   *Why:* For killing Lommy, stealing Needle and the torture at Harrenhal.  
>   *Status:* **Dead.** Arya killed him after a chance encounter at an inn.

> - **The Mountain**  
>   *Why:* For the torture at Harrenhal  
>   *Status:* **Alive-ish.** Qyburn resurrected the Mountain as Ser Robert Strong after the warrior was fatally poisoned by Oberyn Martell.

> - **Rorge**  
>   *Why:* For the torture at Harrenhal and threatening to rape her.  
>   *Status:* **Dead.** Arya killed him after Rorge and Biter attacked her and the Hound.

> - **Walder Frey**  
>   *Why:* For orchestrating the Red Wedding  
>   *Status:* **Alive.**

> - **Tywin Lannister**  
>   *Why:* For orchestrating the Red Wedding.  
>   *Status:* **Dead.** Tyrion killed his father with a crossbow while Tywin was using the toilet.

> - **Melisandre**  
>   *Why:* For kidnapping Gendry.  
>   *Status:* **Alive** and old af.

> - **Beric Dondarrion**  
>   *Why:* For selling Gendry to Melisandre.  
>   *Status:* **Alive.** (Although Beric has died and been resurrected at least six times.)

> - **Thoros of Myr**  
>   *Why:* For selling Gendry to Melisandre.  
>   *Status:* **Alive.**