It's hard to describe Klingons as evil if you don't believe in "evil" as a concept or a something that possesses real existence. As oppose to a interpretation of an isolated act on the part of an individual. If the Klingons are a single people, one culture-nation-mind set, then you can examine the actions of the majority of klingons that we've seen on screen and draw a concussion about the whole. If the Klingons have a total militaristic society, it possible there is no civilian populace. They seem to have achieved 23rd - 24th century levels of technology while remaining a essentially primitive people. They don't just fight, they love to fight, jump at the chance, go looking for it. Early non-canon fiction about the Klingon had them conquering, subjugating planets, enslaving multiple species, a good partion of the crews on their warships were slaves (servator races).
If you don't believe in generally in "right" and "wrong", then sure they're not evil.
If you don't believe in generally in "right" and "wrong", then sure they're not evil.
There doesn't seem that there was an effort on the part of the Cardassians to colonize Bajor in the sense of moving large numbers of Cardassian there to live, as they did on other worlds. Nor any effort to indoctrinate the Bajorians into the Cardassian culture. Carrdassians were not looking for leberstrasse. The Carrdassians wanted natural resources, JustKate may have nailed it with the example of the Empire of Japan's invasion of north-east China. That would make Bajor as Manchukuo and Kubus Oak in a simular role as Puyi.but it also resembles other recent and infamous occupations, such as Japan's occupation of China.
I never saw the gradual adaptation you spoke of. But if the klingon are capable of culture evolution it raises the question of how long they have really been a warrior people. Kahless sometime sounded more like a wondering poet than a fulltime warrior. The Klingon "way of the warrior" may only go back a very few centuries in time. Given that they are such a fixure in star trek, we know surprising little about the overall klingon culture. Far less than we do about the Cardassian or Bajorian cultures.I'd describe Cardassian, Romulan, and Klingon societies as being fundamentally sick because of their imperial doctrines; I'd also describe the Klingons as being far less sick than the others because their culture seems to gradually be adopting the Federation's values as they continue to be allied with the UFP.