The portrayal of the Kree in comics is not straightforward. They have both good and bad aspects depending on the context and the writer's interpretation. Sometimes they act in self-interest that might be seen as bad, but other times they have noble goals.
The strength of the Kree and the Skrull varies. The Kree often have more advanced weapons and a structured hierarchy. However, the Skrull's ability to blend in and surprise their opponents can tip the scales in their favor. It really comes down to the particular circumstances in each comic.
Another type of story could be about a Kree's journey of self - discovery. In the Marvel universe, the Kree are often portrayed as a very regimented and purpose - driven race. A fanfiction could explore a Kree character who starts to question their role in the grand scheme of things. Maybe they encounter a new culture or ideology that makes them rethink their own values and beliefs.
Fanfiction.net also has a large collection of Marvel - related fanfictions. You can search for Kree - specific stories using the search function. But be aware that the quality of stories can vary greatly, so it might take some time to sift through and find the really good ones.
The theme of slavery can be shown through the harsh living conditions of the slaves. For example, they could be housed in overcrowded and filthy cells beneath the Colosseum. Their meager rations and lack of proper medical care can be described. In the battle, their desperation for freedom can be evident as they fight against their Kree masters, who have all the power and resources.