If you remember the episode Kansas,
Zelena, after being defeated and imprisoned, was killed by Rumplestiltskin and her lingering life essence was the last ingredient needed to open the time portal. The time portal opens and sucks Emma, Hook, and Zelena into the Enchanted Forest past.
At this point, he/she is down and almost out, and just wants revenge. She is no match for the Evil Queen of this time and just capitalizes on the opportunity that Emma wanted to save Marian and take her to present day Storybrooke. So,
she kills Marian and steals her identity with the Six-Leaf clover. This actually worked wonderfully for her, because she completely destroyed Regina's happy ending, for a little while anyway.