I love reading all those femsheps background!
I'll try to introduce my Shepard properly without going into an unreadable wall of text.

Margaret Emma is one of those babies born into a taxi, except that you have to trade the taxi with a space ship, and the fact that she never really left this kind of life for all her childhood. As a spacer, she never had what people call "home", even if she likes to joke about space being such. As a military child, family was more about shifting from ship to another ship every 6 months and being either with Dad or Mom than the traditional "Daddy and Mommy with Baby together". And even with that, all was still about work and little time left for being a parent. Add the fact that there isn't usually children in space ships and you gets the picture of almost 11 years of life in an adult world with barely any contact with grounded life.
Then, there were school, boring, grounded school... and boys... which shaped a lonely girl into a rebellious teenager: if she was not good enough for her parents to pay attention to her, they had some headaches with reports from the teachers about last Margaret Emma's exploit on "how to make people go mad in one hour". Marga, as she enjoyed to be called at this time, leaded a dissolute life made by a total uninterest in studying, superficials relationship of all kinds and moving from space station to space station. At 17, it's the final rupture with her parents, she cut all ties, took an apartment with boyfriend, throw the boyfriend out, kept the apartment. Then, after one year of little jobs she never cared to keep, got no money, no diploma worth mentionning, still hate school, enlist at the Alliance. Then, biotic training, pistol training, adept classe, promoted to space marine due to space previous experience, LOVE it, stay in the Alliance.

This reluctant recruit quickly realized that she was pretty good at all this military thing, not the best some said but she has the merit to master her biotics before a lot of her promotion and well, if she wasn't an emphatic leader for the moment, it would eventually come with time. So, she was assigned in the squad of marines sent into Akuze for investigating the sudden disappearance of all signals coming from the colony, the only biotic among them.
Shepard's report said they landed at nightfall near the desert colony, effectively, all comm' system were dark, the AA guns protecting the colony were disconnected as well... so, the team agreed on staying for the night to fix it and beginning the research at dawn. As Margaret couldn't do anything about tech, she went out of the colony to see the countryside and smoke one or two cigarets... but she had to return in hurry to the colony because of the threwsher maw's overwhelming force was destroying the buildings where the team assigned to reestablish comm link were. The public know the vids in which Shepard is recognisable by the blue aura of her biotic barrier, trying to escape the maw' spit of acid, the tentacles trying to grab her without forgetting the slaughter of the others marines. But they only seemed to remember that it was she who actionned the AA guns, repaired in hurry during the attack, and made the threwsher maw leave.
When Margaret Emma think of Akuze, she remember when she awake in a hospital bed, when she looked at the reflect on the screen, at the face of a woman staring at her with a tumefied face. Then, the doctor came. He told her that she was in coma for several days and she had severals injuries, both internal and external, the most important being a head trauma. She was picked up by a rescue team half dead, warned by a beacon activated by herself before falling inconscious and there were nobody else who made it back. He was sorry. Margaret stayed a long time at the hospital. Her mother visited her, brushing off their old grudges. Her father didn't. Hannah Shepard was really supportive toward her daughter future recovery but sooner or later, she had to return to her duties as officer. She never accepted that Margaret Emma was changed forever, that she couldn't do it alone, that she wasn't able to read mails anymore.
Everybody doubted her aptitude as being able to retake her job as before, she lost too much, they said, there is no way someone like her can be expected to be able to be a good officer, they said. Some months after being out of hospital, she was asked to join the N7 program. Some years later, Margaret Emma Shepard was the first human Spectre.

I hope this can give you an overall of the character pre-ME, as it build roots for the "Why" of her today's views. Alas, I think I failed at the "not-too-wall-of-text" part so, I will not develop the ME part.
Roughly, for all the three games, Margaret has 100% Paragon with like 5-10% Renegade explained mostly by clumsyness (I never succeed at keeping all the colonists alive). Most of it because, since Akuze, she value life above all, incounsciouly, it might be a way for her to have some kind of pay-back for all the lives lost on Akuze. In ME2, because, funnily, a lot of paragon dialogue options were about saying that Cerberus sucked and was evuul, so, it fit totally the character

. Also, since Akuze, she adopted a more friendly attitude toward people because the long time she passed alone in hospital made her realized how empty was her life and how difficult it was to port when you have no reals friends. And her alexia obliged her to speak to people to know them instead of reading their service record, reading VI are so boring in fact.
So, in terms of game mechanism, her "alignement" didn't really change but the reasons which motivate it does (so, it is the scenario "all is in my head").