Umm… do you mean ethically, or in terms of magic (or other) powers? Well, we know he can appear in Guen’s dreams when he hasn’t even been born yet, so that would suggest some ability to transcend time, and to get into people’s heads. Traditionally, he’s capable of killing Arthur and destroying the kingdom.
Yeah, his reaction is pretty extreme. He’s not a big fan of formality or the concept of noble/royal bloodlines. Remember, he’s adopted. As more of his backstory is revealed, his reasons will become more clear. One of the reasons Arthur likes Lancelot so much is that Lancelot has always treated Arthur as a person and a friend rather than a king – but not everyone thinks that’s appropriate, of course.
That, he felt like he deserved. Not so much mellowness (though he is pretty laid-back, in some ways) as guilt.