Imperial guard use flak armour, not carapace. Unless she comes from a storm trooper regiment, in which case you may ask your GM permission to have a hellgun instead of a boltgun in your starting equipment, because those are standard issued in stormtrooper regiment.
And really don't leave home without a good-craftsmanship force field.
Next, you need to fill out your character's motivations. Did she shed the burden of the imperium solely out of self-preservation ? I ask because storm troopers are typically very dutiful and would rather die than be traitors.
The Renegade archetype is all about revenge. To a renegade, the crusade is personal. Maybe she knows of a powerful Imperial official who betrayed her or her kin and she wants to see all of that nobles works to crumble to dust as she dances naked on the ashes of his charred-down body.
Madness is also part of any black crusade character (hence why they are not affected by insanity points : they're all aldready insane!) so you might be interested in thinking about how that madness manifests. Are her premonitions about her possible future real, or are they mere lies and illusions ? Or perhaps are they the confusing manifestations of a slaneeshi love-poison slowly corroding her mind and her senses for a dark and sinister purpose ? Will it all be for the good or for the ill of your character ? Where chaos is involved, nothing is certain.
Work hard on your character motivations in order to "get" her. Why is your character sowing corruption, why does she does that ? Why why why why. If you haven't figured it out, why is the most important thing you can ask about your character. Write out the answer, read them to your GM and fellow players and see how that sounds.
Let me tell you a story about one of our play experience : once, we played a medieval game set in our interpretation of Mount and Blade's Caldradia.
The full recalling is here, but the important part follows un-hidden.
We spent the first session planning, as a group, the whole campaign in advance, where one of the characters would be a prince claimant to the throne and how the other characters would be his companions, one had the money, another one had the army, and a last one was a deceitful liar. We then established that the Noble with the army was in fact a gambler, and in much debts, and that he was following the prince in the hopes of getting a reward that would clear his debts, the rich merchant was also a gnostic - a historical heresy - that wanted the kingdom burnt to the ground in vengeance to the church, he supported the princes' claim because he felt the prince's extraordinary incompetence at ruling could accomplish just that. The last one was in fact a foreign (Rhodok) spy attending the court of the current king and he supposed that the princes incompetence would he get to the throne would create such political turmoil in the kingdom that a glorious republic like that of his own nation could emerge. Also the prince himself only wanted the throne in order to marry his cousin, because we made our fictionnal kingdom a matrilineal patriarchy : you get to a social position by marrying the daughter of the wife of the guy who had it before you, but it's the man who actually exercise said position. So the overarching goal of the campaign is to have the Pope pronounce the marriage of the prince and his cousin.
Then we divided the play in five arcs, each focusing one one of the characters gifts and his flaws. The first arc was about the prince rallying support with his charisma and luring traitors with his guilliblity. The second was about the merchant using his money to bribe his agent in positions of power within the court of the current king, while planting an heretic or two which, if discovered, who make the church, herm, "reluctant" in pronouncing an eventual royal marriage for fear of condoning heresy. The third act was about secession and warfare, with the Noble getting glorious victory with his armies and his skill at using the confusion among the loyalist rank in the second act. It was also at this stage that the extent of his gambling became apparent, culminating in the group failing to pay their soldiers and the mercenaries "taking pay from the land" before disbanding. In the fourth act, the spy would reveal it's true allegiance to the other characters, and inform them that his sponsor had gratiously offered funds for the rest of the campaign, that where put to good use and eventually won them the war. At the end of this act, the spy character was to be murdered by the noble character in order to prevent a foreign country from infeodating the whole kingdom. Althought legally dubious, the nature of the spy's work would turn out to make it impossible for the foreign ruler to back up his claim with evidence. The fifth act had the characters successfull in their rebellion and further turning on each other as their agendas started to diverge.
The important part is that it was all decided with the group, in the first session. It made all the players very engaged in the story, and, althought there were not any spoilers, it gave a real feeling to the players that their characters were part of the world created by the GM, something more than I have ever been able to replicate since even by using the established canon of 40K. It was genuine roleplay. It was awesome.
So suggest you take a minute to talk to your GM and your group how each characters fit, not in the campaign, but in the story.
Hope your experience will be enjoyable.