Deyan Deyanov apparently drew a finger across his throat as he made the chilling threat in a shop on the holiday island.
The 28-year-old Bulgarian is believed to have stormed from the mini-market in Los Cristianos after he was ordered to leave.The shopkeeper, who gave his name as Carlos, said: "I asked him what he was looking for and he held his hands far apart and said: 'I'm looking for a knife this big'.
"I asked him why he wanted it, and he said, 'I'm going to kill somebody'. He drew a finger across his throat to show how he would do it."
It is thought that Deyanov had previously received psychiatric treatment.