Antagonist is an artist, a painter. Finds a meteor and hides it inside a teddy bear. It has some psychic influence on him and I believe he ends up killing his girlfriend and painting with her blood.
1 Answer
I believe this is "The Hungry Eye" (1959) by Robert Bloch. The only detail you have wrong is that the "girlfriend" (just a pick-up, really) is the killer who uses blood as paint.
The narrator is Dave Larson, who makes a living as a stand-up comic in Chicago. One day he suddenly sees his brother George, who immediately runs from him but later shows up at his comedy show with a stuffed pink panda. (He hasn't seen George in five years, and that was in Boston, so this is unexpected.)
George is a suspect in a murder that took place at the home of a recently deceased collector of "oriental art," and begs Dave to go somewhere they can talk in private about something that's worse than a murder. Dave has to work, and leaves George at his table where he is picked up by Sarah, a club regular who is an artist.
A Dr. Shotwell, who is managing the collector's estate, shows up at the club to talk to Dave and warn him.
"All I must tell you is that I think amongst Chandler Harvey's collection of rarities was an unusual meteorite—an ancient, jewel-like object which, in a certain sense, is alive. And I believe your brother must have found it today."
But Dave is skeptical:
I stared at him. "You mean to say you believe this meteorite possesses some intelligence which influences men to kill? But why?"
But he goes to Lucy's apartment/studio because he knows he needs to warn George. He finds Sarah painting an impressionist canvas of the pink panda:
Her palette was the body of my brother George, who lay sprawled out on the couch, his limp arms still hugging the pink panda to his breast. From breast to crotch he’d been ripped completely open by her palette-knife, and she was dipping her brush in his wound, dipping her brush in blood and entrails as she painted her monster from life. From his life—
Dave grabs the pink panda and runs, and finds that George had hidden the meteorite in its head:
George had cut it, and not because of intoxicated impulse. He'd cut it and carried it along with him, and no wonder it thumped, because the meteorite was inside. That's where he'd hidden it away. And he'd carried it to the studio and Sarah was aware of it, and then—
Naturally, this being Bloch, the story goes downhill from there.
The story was originally published in Fantastic, May 1959, and you can read it at the Internet Archive.