Hm... I don't care about definitions. I decide for myself if I think that someone is evil or tyrannical or not.
For example, for Big Mom I would say yes to both. Even though people in Tottoland seem pretty happy and the biggest danger they face seems to be getting caught up in one of BM's binge-eating/hunger attacks (which would make it one of the safest places in the OP world, ironically), she acts as a tyrant towards her crew and family and does tons of blatantly evil things.
But if you ask me about characters like Whitebeard, Shanks, Roger or Sengoku, then I wouldn't be able to give a definitive answer.
It seems like WB was a decent person, but was also capable of hurting and dominating his rivals. The only place where I really questioned his moral standpoint was when he defeated Ace's pirate crew and insisted that he should join him, even though he didn't want to. He used his strength to get what he wanted and didn't take No for an answer, which is pretty tyrannical imo. On the other hand, I don't remember him harming anybody who didn't challenge him. So I'd say tyrant? He may have had a little of that, but I don't think he conquered territories and hurt the people there, so not really. Evil? No, there is no evidence of him doing evil things... unless you count hurting or even killing (don't remember if he did that) other pirates in pirate battles.
Which I gotta say, I don't really count killing an evil pirate as an evil act. XD
I know people were always fighting about that in DnD too. "You can't kill this orc, that would be evil!" - "But he attacked me!" - "But still, you're a paladin!", etc.
I'm more pragmatic when it comes to stuff like that.
So on to Shanks. We know so little about him that it's almost impossible to tell what kind of person he is. I see him as a neutral character. He doesn't harm people, but he doesn't seem to be a hero who travels the world saving people either. He did end the Whitebeard vs Marines war and probably saved many people by doing that, but other than that I don't remember him doing anything noteworthy. He just seems to hang out with his crew and chill most of the time. Maybe he has some hidden ambition that we don't know about, but otherwise I find him a bit of a boring character so far. >.<
About Roger, I dunno. Most of his former allies we've seen or heard about (e.g. Rayleigh, Momonosuke's father, Shanks) seem to be decent people, so he can't have been a bad person. Then again, he's probably more of a "neutral" character too.
What I really like about OP is that there are very few characters where you can say: "This guy will always do good things." or "This guy only does evil things." They are so interesting because they are very human-like, sometimes doing things for the sake of others, sometimes doing selfish things, sometimes wanting to do good but doing a bad thing, sometimes acting on a whim, sometimes regretting their actions.
Sengoku is a character I don't understand at all.
While he has done good things and seemed like a good person, he has spent his life working as the head of an army that does about as much good as it does evil. Why would he do that? If he was anywhere near a decent person, how could he serve the tenryuubito? I hope that we'll get an explanation at some point, maybe when Strawhats vs. The Government is on (probably towards the end of the manga xD).
Akainu, huummm, yes, he seems very evil. I don't think he cares about the lives of civilians at all, but I don't know what he DOES care about. He must have some sort of motivation in life, right? I guess he might be a conservative kind of person. "Things have to stay the way they have always been. The Marines will make sure that order exists in the world and lawless people can't do as they please." Where I'm pretty sure that he doesn't make a difference between actually harmful people (most pirates), people who want to just live their lives and follow their passions (e.g. SHs, those Oharan historians) and people who want to change the world for the better (Revos). Any rebels must get cut down! Like that rebellious dandelion growing on the edge of a perfectly groomed lawn.
