**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 (list more or less copied from [here](http://www.tvguide.com/news/game-of-thrones-arya-kill-list/), updated with status **including spoilers from S8**):

> - **The Hound**  
>   *Why:* For killing Mycah, the butcher's boy.  
>   *Status:* **Dead.** Cleganebowl.

> - **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:* **Dead**.

> - **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** - as far as we know.

> - **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:* **Dead.** Cleganebowl.

> - **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:* **Dead.** As of the Season 6 finale Arya used her assassin skills (including face-changing!) to infiltrate Riverrun and kill him

> - **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:* **Dead.**

> - **Beric Dondarrion**  
>   *Why:* For selling Gendry to Melisandre.  
>   *Status:* **Dead.**

> - **Thoros of Myr**  
>   *Why:* For selling Gendry to Melisandre.  
>   *Status:* **Dead.** Mauled by undead polar bear.  Succumbs to wounds and cold.