why joon want to shoot the girl in the beginning in it's war and what is their relationship??

First, Cinta this song, and currently listening to it~♥.
Second, Here's the story line:
Lee Joon is a secret agent whose supposed to kill that girl, but couldn't because he began to have feelings for her. Then his partner (?) had shot her while Joon and her were running away, causing Joon to kill his partner and run away to Thunder, who Joon asked to watch her for a while. Thunder and the girl then began to tunjuk some feelings to one another, and when Joon came back, Joon noticed all of it, and took the girl away. Thunder broke off Joon's hands off of hers, causing Joon to punch him. Thunder fighting for the girl, punched Joon back. Joon then brought out a gun, making the girl cover Thunder so she would get shotten ( Is that even a word? Keke ) but instead, Joon shot himself to get away the broken heart. Joon was actually planning to escape with the girl, but instead gave it to Thunder and the girl.
And the lyrics in the song:
{ Mir }
Engrave this in your ear, I will never leave anda alone.

Probably means that Joon's death will always haunt Thunder.

Welcome~~!!

he's too angry because of his jealousy, their relationship is boyfriend and girlfriend (at the end have a long distance relationship)