That unfortunate association - if you're an accountant - comes from the Russian verb likvidirovat. It was a favourite word of Lenin, even before the revolution gave him the chance to put it into practice. Previously, 'to liquidate' had been free of sinister overtones.
It came from a secondary meaning of the word 'liquid', which had entered English in the 14th century. A liquid is wet but tends also to be transparent.
And in the 17th century, 'liquid' came to describe a clear or transparent argument. The financial meaning followed. A liquid debt was one that was undisputed. Such a debt could then be dealt with, or 'liquidated', a usage first recorded in Dr Johnson's dictionary of 1755. The 'liquidator' arrived 100 years later to enjoy half a century of respectability - before Lenin came along.