He didn't stop screaming the whole time she fed.
You talk to anyone who works on crime scene, or in forensics, and they'll tell you that you can define if blood is arterial in seconds. The thing is with arterial blood is it's under immense pressure as it is pumped round the body. All blood is under pressure, of course, but especially the bright red, oxygenated stuff in your main arteries.
As she ripped into Janet's neck, it sprayed everywhere. There was blood, sticky and hot and metallic in taste; and it covered every surface. It covered Robert as he desperately tried to prise his lover's throat from his wife's maw. It dripped and gushed and stained as Betty made those unholy, disturbingly intimate suckling noises. She gulped at Janet's neck like a child at it's Mother's breast.
No, not she. It.
Eventually, after a panicked blur that could've lasted seconds or minutes, she let the corpse slide to the floor with a dull thud. A blush painted Betty's cheeks. She'd been wearing white before, but now, she was all red.
She glided to the sofa, and softly sat down, watching as Robert cradled Janet's lifeless corpse. Tears drew clear lines in the blood on his face.
"Why? Why Betty," he sobbed.
Her face was amused. As she spoke, her fangs were impossible to not notice. Blood was smeared down her chin, onto her neck.
"Robert, she took you from me," she chided. "I had to do something- besides, I was hungry. You'll understand soon enough."
He pressed his face into Janet's saturated cardigan. Red flowers of liquid began to bloom on his shirt.
"Robert, don't be sad. You have me," she whispered. She went to him and planted a kiss on his cheek.
"I-I...How did you know. How did you find us," he gulped between sobs. "I have to know."
She laughed softly. Oh, how she could change from an animal to an angel in minutes.
"You left your phone with me, remember?," she traced a line of blood on his face with her thumb, and stared lovingly at him. He didn't reciprocate the eye contact.
"Don't think it's your fault. I suspected for a long time, so long in fact I went out one night hell-bent on getting revenge. The idea was to find someone else, do a bit of heavy petting, make you jealous...". She sighed, looking up. There was almost nostalgia in her eyes.
"And then, you got...bitten...," he finished.
"Yes. Typical, the one time I try to find someone else, and I find a vampire and end up dead in an alleyway. It was confusing, waking up on that slab."
Robert wasn't scared anymore. He was too tired to care for himself.
"I think I knew, from the start. When I ran home from the Morgue the sunlight burnt my skin. I was almost in flames before I got in through the door. Then I looked in the mirror...or more, tried to. And then, I knew."
She lean her head on his shoulder. He shied away.
"Then, of course, Pete came to the flat yammering about how I was the spawn of Satan. He had to go."
His stomach knotted into a pretzel. Poor Pete.
"How did you get here," Robert said, his tone monotonous. He stroked Janet's hair.
"I'd traced her messages when I saw them on your phone. Had to wait for the sun to reach the right place to make a shadow long enough for me to get to the car, though. It's hardly fair on these bright, summer nights."
"Come on Robert. You have to forgive me sometime."
He said nothing.
"Robert! I'm your wife. You said you'd love me forever, and I know you will, I'll just have to teach you to. I'll change you, and then we'll-"
Her words kicked him into sitting position.
"You're going to...change me?"