Render the bacon until crispy in a Dutch oven over medium-high heat. Remove and set aside on a paper towel-lined plate. Add the onions and bell pepper and sprinkle with salt. Stir to combine and saute until translucent, about 5 minutes. Add the kale and red pepper flakes and toss to combine. Allow the kale to wilt, about 2 minutes. Turn off the heat and toss in the apple cider vinegar. Taste and adjust the seasoning with salt and pepper.