Najara is considered the Joan of Arc figure in Xena warrior Princess. She and her merry men fight evil and stop thieves, murderers, rapist and slave traders from killing and imprisoning people.
When it comes to skill, she was stronger than Xena and defeated her in a fair 1 v 1 fight. It should be noted that Xena only defeated her in the rematch by making her unfocused during their fight via putting Gabriel in danger.
What caused Najara and Xena to clash is that she would round up any surviving prisoners from battles she engaged in and gave them 3 days to reform their evil ways. If she found they were not sincere or that they were unwilling and would continue doing harm to people in the future, she would execute them. This caused Xena to consider her way of doing things wrong and the two clash. Keep in mind, Xena actively kill evil doers in battle without judge, jury or executioner but seems to have a problem with this woman's method of executing prisoners of war.
After escaping prison and being seen in a follow up episode, she is shown to give up using the sword and is living a peaceful life projecting the love of Eli. Xena stumbles upon her when she is getting kicked in the chest by a warlord that was going to rape and sell a bunch of women. The women claim she was defending her and even though she was capable of killing every man there, opted to never lift a finger and took a beating.
Xena upon seeing her decides that she should go back to prison even though she is badly beaten up. Keep in mind, Xena has killed thousands of innocent men and women and implicated in death and murder of children but she never once did prison time for her actions. Yet, she wants this woman to rot in jail for essentially killing "bad people".