He had found deep shadows to hide in. An abandoned jewelry store stood next to the gym. Its windows were boarded up, but its deep entrance made a good place to stand and wait. There was a blanket lying in the doorway, left there by someone homeless perhaps. He stood silently in the shadows and watched.
The gym was closing, and a few stragglers were coming out into the parking lot. Buff young men passed him by without noticing him. Toned women in scanty clothes walked by without sensing his presence. He was well hidden.
The last one came out the door. He looked around the parking lot and saw all the others had left. He studied this girl carefully. She was a beautiful blonde, with nice boobs under her t-shirt. They were not large enough to require her to wear a sports bra, and obviously she had chosen not to. They bounced slightly under her thin shirt and the nipples were clearly visible. Her shorts were as short as they could possibly be. There was a nice gap of bare midriff between the shirt and the shorts. He found her very attractive. Yes, she would do nicely.
She walked quickly past the dark doorway. As she passed he glided out of the shadows and grabbed her from behind. One hand went over her mouth so she could not scream, and the other reached around and grabbed her boob. She seemed to panic at his touch, dropping her purse and squirming under his grip, but he was strong. He pulled her back into the dark doorway. Still keeping one hand over her mouth, he groped in his belt and found the handle of the knife. He pressed it into her throat.
She stopped struggling at that, afraid of hurting herself. He hissed in her ear, "Make one sound and I cut your throat." Then he carefully released his grip on her mouth. She whimpered softly but was afraid to scream. The knife stayed at her throat, and the hand that had covered her mouth moved down to squeeze her nipple through the thin fabric. Slowly he turned her around and she saw he had a mask over his eyes.
He put one hand on her shoulder, and with the other reached out the knife point toward her. Slowly, starting at the neck, he cut down through her t-shirt. The soft fabric parted to reveal her tits under it He stopped for a moment to enjoy looking at them, and to enjoy her trembling as he did so. She whispered, "No, no!" but he jerked the knife toward her throat and she quickly was silenced. He cut on down through the fabric and her boobs were fully revealed.