Okay, so I dreamed this happened and I thought it would make an interesting fic to explore so... enjoy! :)
"I know you didn't mean to kill her, Chloe," Beckett spoke calmly, clearly, whilst trying to ignore Castle by the door. Why wouldn't he just follow her orders? She couldn't risk him getting hurt, not when he has Alexis... The girl would never forgive her. Brief visions of anger from the 15 year old clouded her head, imagined the rage Alexis would feel for the detective putting her father in danger. "He's a writer, not a cop," she could almost hear the red head spit the words at her. "He was your responsibility, he trusted you to look out for him!" she would scream, tears streaming over the pale skin of her face as she brushed a lock of her flaming red hair out of her enraged eyes.
No. No, she couldn't let anything happen to Castle.
"She was my friend, you know? But she just... She didn't... She didn't get it," the detective caught what Chloe was saying as the knife she was holding cut deeper and deeper through her jeans, into her bloody skin. The crimson liquid was now oozing rapidly out of her self-inflicted wound and Beckett took another deep breath. She just needed to get everyone out of this alive.
"She didn't get how we felt about each other," Chloe continued. "So when... when she turned around, I just grabbed the bleach, and I hit her with it. And she fell. And I was so scared, I didn't... I didn't know what to do, so I j... I put her in the dryer. I was so angry!" the woman practically roared, her short brown hair now hiding her face. Beckett shut her eyes at the outburst wanting to make herself as small as possible in her vulnerable couched position on the floor. Kate still had her glock in her hand but she wasn't intending to use it unless she felt Chloe was a threat (or unless she needed to remind Castle to stay where he was). But at the moment it seemed like Chloe was calming down, only getting her emotions out. Not a threat. Yet.
"You know, just so mad," Chloe finished.
It was now or never.
"Chloe, put the knife down, okay?" Beckett spoke, trying to keep her voice steady, although in this current situation it was proving tough. "Let me help you," she detective looked the nanny straight in the eye and Chloe looked down, beginning to lower the knife. "Just put the knife down," she tried again and the knife was almost released. So close. "Come on."
"No." she spoke suddenly.
No? Did she just...? And the knife was raised again, holding the hilt steadily with a firm grip. Castle's heart was in his mouth as the nanny marched towards the detective, a determined gleam in her eyes. Was she about to-?
It happened to fast, too quickly. Beckett was unable to lift up her weapon fast enough, and before Castle knew it the knife was plunged into Kate abdomen causing a horrifying groan to tumble from her lips and her pupils to dilate in shock. The knife was withdrawn from her body slowly generating another groan from the detective as she dropped to the floor.
Castle stood by the door in terror, trying to get his head around everything that just happened. He had to get to Beckett, had to apply pressure to her wound but there was one problem: Chloe.
She stood over Kate's body, knife still gripped in the palm her hand, snarling through her teeth and breathing heavily. The nanny turned slowly towards Castle – nothing was going to stop her now, nothing, and she proceeded towards the writer weapon wielded above her head, his breathing becoming erratic. She was almost there when...
Bang.
One shot to the back of the neck from the fallen detective and Chloe too fell to the floor.
"Detective Beckett!" Castle shouted as he ran over to the detective. He kneeled gently next to her, careful not to hurt her and put pressure on the wound, earning another groan from Kate.
"Come on Beckett, come on. You're going to be fine. Come on," but he was losing her quickly and her eyes soon fluttered shut.
"Kate, come on, look at me. Come on please, please. Stay with me, come on Beckett," and Castle supped her cheek. "Come on! Please!" and desperation entered his voice. Tears were starting to fill his eyes. No, she couldn't leave him. He only just met her, she couldn't. Not now.
With that Ryan and Esposito burst through the door, took a look at the distraught writer leaning over the cop and Chloe bleeding out onto the floor.
"We heard a shot," shouted Esposito as he and his partner rushed to Castle and Beckett.
"Shit," murmured Ryan as he saw the blood on Castle's hands and pulled out his phone dialling an ambulance as fast as he could.
"Chloe..."Castle began but the words stuck in his throat, unable to speak.
"It's okay bro'," Esposito said as he patted him on the shoulder and bent down next to Castle to check Beckett's pulse.
"Her pulse is still strong, keep applying pressure," he told Castle and before the three of them knew it they heard sirens outside.
The paramedics swooped in with an assortment of equipment and took the bleeding detective out on a gurney, each one of the paramedics screaming a string of words at each other in distress, trying to fulfil their individual roles. The writer almost spoke out and demanded to ride with Kate to the hospital, but it was too late. The siren was howling as the ambulance drove away from the building, diving into the oncoming stream of traffic, but all Castle do was stand there and watch with the other two cops. He couldn't help but wonder whether that would be the last time he would see her.
He quickly pushed that thought away as Esposito and Ryan led him into their Crown Vic and followed the ambulance to the hospital. He hoped he would see Detective Beckett again soon.