@kaminomikan
She just found out her brother died and that Arga knew. You don't need any info as to why she'll act like that, just basic knowledge of emotions.
Among the things people tend to do at the death of a loved one:
1) Become depressed.
2) Seek vengeance. (if said loved one was killed)
3) Seek someone to lash out at. (which in this case was Arga)