He wrote half a dozen letters while on the run and burned them all before completion. Karkaroff couldn't afford to communicate with anyone; owls were too easy to track, especially over long distances. Still, he had things to say, and sometimes he put quill to parchment and tried to say them.
Three letters were to his colleagues at Durmstrang, explaining. He needed them to understand.
Two letters were to Viktor, apologizing. He wasn't sure what for, but he was certain that he needed to.
The last letter was addressed to Severus, but he soon realized there was nothing to say.