The Skrulls weren't initially evil. Their modus operandi used to be to transform themselves into whatever new races they encountered, with the intent not to conquer, but simply to foster trade and relationships with these cultures. But when they landed on the Kree homeworld, the Kree were still co-habitating on their homeworld with another dominant race, and the Skrulls...didn't make good decisions, lol. They chose to favor the other race to share their knowledge and technology with (after putting both races through their own special "tests" to determine which was more worthy), and the Kree took offense and rose up and attacked them, stealing a lot of their tech and proving themselves a formidable threat. The Skrulls re-fashioned themselves into a warrior society and the animosity between the two worlds grew and basically became all-consuming. The Skrulls only came to Earth because of its strategic position in the galaxy relative to the Kree homeworld. That's pretty much all they care about at this point.