He holds two principles above all else, those of working for the greater good and self sacrifice for the good of others. He practices what he preaches as well, believeing his purpose in life to to be an agent for good. This belief began when he got his cutie mark, by stopping another pony from being beaten up by taking most of the blows himself. Due to this he came to the conclusion that he had a talent for self-sacrifice, especially when working for noble causes.
He has taken Rarity as his Deity to reflect these beliefs, going so far as to use her preferred weapon, the rapier, in combat. He will always be among the first to jump to the aid of others, largely because he often forgets or simply decides not to assess the situation. However he is well intentioned, and is fully invested in his belief that he is meant to be a hero. This often leads to lengthly dramatic speeches on subjects he believes are important, usually to do with good and evil.
While all of the above is true, there may be more to his story. The following is part of a transcript from the trial leading to his ecommunication from the Argent Legion, an organization working to promote harmony in Equestria.
"Cerulean Shield, do you know why you are here today?"
"I assume because you've realized that Im doing good on this organization and wish to apoligize for constantly punishing me the actions I've taken to better Equestria."
"Mr. Shields, in your short time here at the union you've mercilessly slaughtered more than a dozen largely innocent ponies, who were all highly co-operative with our organization and our goals. Do you wish to offer an explanation for your actions?"
"Allow me to ask you something, your honor. Is it not the goal of this organization to promote harmony?"
"No one is arguing the goals of this organization Mr. Shield. Would you kindly answer the question."
"I submit to you that every pony I injured was in fact working against the goals of this organisation, by partaking in activities likely to cause conflict and therefore disharmony. In order to prevent this I took the logical step of removing the source of conflict and preventing the potential problem."
"Mr. Shield, just last week you murdered one of our vest informants for absolutely no discernable reason."
"OBJECTION! I personally witnissed the pony in question dealing with known criminals and miscreants. He was clearly engaging in and plotting illegal activities, and I took the necessary steps to prevent these activities from happening."
"Objection overruled. It was that pony's job to plot illegal activities, he was an UNDERCOVER informant. Several witnesses have told me that you wre fully aware of this fact, and yet you still attacked him with little to know warning because you claimed he was 'evil'. How can you possibly justify that?"
"I am only working for the goals set forth by this organisation. He was an evil pony highly likely to cause disharmony, and so I eliminated him."
"That's another thing we need to speak to you about Cerulean. Almost every one of your attacks has been because you believed the victim was 'evil'. Ignoring the fact that you are both far to quick and too harsh with your obsession with removing 'evil' ponies from the world, your standards for what constitutes an 'evil' pony are ridiculous! You can't kill somepony just for making a few poor choices, and unless you adjust your standards you will have no place in the Argent Legion."
"Your honour, I have only two principles which I want to work for. I wish to promote the greater good, and I wish to defend those weaker than myself no matter the cost. I have upheld both of these principles time and time again, and I decided to join tis organization because you seemed to share the same goals as me. I will continue to uphold these values and removes anyone who works against them with or without the backing of the Argent Legion."
"Then I have no choice. Cerulean Shield your membership in the Argent Legion is hereby immediately and permanantly revoked. If you attempt to return you will be forcibly removed from the premises. You will operate as any normal citizen of Equestria is permitted to, and you will stop assaulting innocent ponies unless you want to risk us to consider you evil and assault you. I wish you a good day."
"Fine, I'll go. You can stop me from working for you, but you'll never stop me from working for good. I will continue to uphold my principles until the day I finally fall, and I can only hope that day will be one in which we have finally achieved harmony. Good day to you, your honour."
The transcript paints a picture of a pony who, although well intentioned, is both quick and brutal in his punishments, causing many innocent ponies harm. Nonetheless he fully belives he is destined to be a hero, and that his actions are fully justified. This combined with his tendency fo self-sacrifice makes him a powerful force in combat, although the combat is often entirely unecessary to begin with.