Is it possible to deny salary if owner didn't like the quality of work?

Hi, I interned at a startup for frontend developer for 6 months, January was my last working month as I had my college exams and this startup still hasn't transferred my unpaid salary for January, I called the owner today and he was saying that he will not give salary because he did not like quality of work. He had fired 3-4 people citing the same reason in previous months and they still have not received their salary. I told him that I will proceed legally and he threatened me that I can call whosoever I want and in return he would mail my college to block me from placements and will not give experience letter What should I do?