Better late than never, I guess. I don't think there's a way this can be done without hiding something, because otherwise, why would you have the character in the first place? The whole reason your hate turns to sympathy is because the story has revealed a bit more about the character in question and what they are going through.
Anyway: (I'm too long-winded, and as such, tried to cram way too much into 2 paragraphs. I expect nothing better than a C- on this assignment)
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Sister Clair tossed the empty gasoline can aside, having finished her circle around the orphanage. She gazed up into the foggy night sky for a moment before producing a small lighter. Sparking it with a flick of her wrist, she tossed it toward the building without so much as a second thought. The flames circled around and quickly rose upon the saturated wooden structure, just as she had planned. A dozen child-like screams of agony accompanied the sound of the now raging fire before her, but still, she had to be thorough. Taking hold of a large metal bat she had brought, Clair made her way to the building's only entrance. It didn't take long, as there she could already hear pounding and fumbling with the door's latching mechanism. The door flew open, and three small figures rushed out, desperate to escape the ever-growing inferno. Clair struck the first one squarely across the head, and then kicked the poor child back into the building. Quickly spinning around, she caught the second by the arm and heaved the frantic orphan through the doorway to join the first. The third hadn't gotten far, as she quickly caught up with and grabbed this one, too. The frantic young boy fought madly to get away with everything he had, but the nun was far too strong for him. He too was thrown into the now-roaring, collapsing inferno. Clair pushed the door closed, and propped her bat against the latch in such a way that it wouldn't be opened again.
Burn, filthy creatures!
Exhausted, Clair knelt down to catch her breath as the screams began to die down and the building continued to burn. Finally, the last of the plaguetouched had been destroyed. She rubbed her arm, hoping to drive away the numbness that had began to appear. Perhaps she sprained or twisted something with one of those throws? As she glanced at it to check for any bruises, her face grew white with horror. A patch of skin had begun to turn green, with a small set of teethmarks at the center.
How did...? Clair had been part of a gathering of townsfolk who had set out to contain the plague of undeath the only way possible--by destroying everything affected. She had been the last of this group to survive, but now, she was going to become the very thing she had fought to destroy only moments ago, making it all for nothing. She couldn't let that happen, no matter what the cost.
I feel so numb, so cold. God, why has such a thing happened here? Help me, please! She sighed, not expecting a response of any kind. Only yesterday, she was naive enough to have prayed for a miracle, but she knew her god wasn't here to listen. Her eyes wandered around the remains of the ruined town and the burnt remains of countless bodies.
How could my God have ever been here? Several tears trickled down her dirt-covered cheeks as she slowly approached the burning remains of the building.
Has the cause I dedicated my entire life to been a waste? Has it all been a lie? As the flames swept around her, a chill rippled throughout her body, and her sight began to fade. There was no light at the end of the tunnel waiting for her, only infinite darkness.
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This would've made a fun (if not somewhat cliche) short story to write. Hmm!