He is ruthless, for sure, when he thinks his actions calls for it. It's quite ruthless to lie and decieve your brother the way he does, but he justifies his actions as them being in everyone's interest. However, if there's any time he should waver, and we didn't see anything like that, it would be when he speaks to Frigga. I got the feeling that he was so wrapped up in what he was doing from the moment he let those Frost Giants in to the weapons vault, to the very end, that he kept up the act even to her. It's clear it's his theatrics that fires up Thor, but I got the feeling he was acting in front of Frigga as well (the scene that was cut, that is). Of course he has something nagging in the back of his mind that this isn't really okay, but he ignores that voice, and in the end he has lost it. Only the lie remains.
Lack of restraint is what characterises both brothers from the beginning, but while Thor changes for the better, Loki changes for the worse. Loki's change is all on him, since he is responsible for his own decisions. No one forces him to make them. Thor is sent to learn a lesson, and he learns, a lesson Loki should have learned as well. When he arrives on Earth in Avengers his head is so full of ****, that he can't see straight, which naturally leads to him being defeated by the Avengers. Never underestimate your enemy, yet he does so throughout the film. Arrogance is the downfall of anyone seeking power, and he has it in spades. Hell, he has a garden full with carefully nurtured plants of wrath. This is what makes the continuing story interesting, what will happen now?
I really have a thing or 50 to say about Thor as well, but after reading the Chris Hemsworth thread I decided to not venture there, because I don't want to come at odds with those who love the character Thor (I like Thor a lot, but it's not all black and white to me). That's one of the difficulties when discussing heroes and villains; some will look at an opinion and say, in Loki's case, oh you're making excuses (which I'm not - he has to own his own decisions and actions) or, in Thor's case, a hero is a hero is a hero no matter what they do or in what name and no one should dare to say otherwise. Which I would, and I think it's a lot more interesting too. What makes a real hero in my eyes is someone who are willing to challenge him- or herself and what they are doing and learn from the past and constantly try to do better. Loki obviously isn't, so he's not my kind of hero.