Alexander's strategy to display the heads of his enemies was a tried and tested technique, with many examples not only in this time period but also in Alexander's previous life.
With perhaps one of the most seasoned practitioners of this tactic being the Mongols, who had most famously built a literal pyramid of skulls of 80,000 men, women, and children when they slaughtered the entire Khwarazmian empire to the last man,
It was said that that pyramid lasted for nearly a century and the stink spread for miles.
And as horrifying as it might sound, Alexander's initial version of the message was indeed similar to that one, composed of trains of carts filled with heads of the entire city's population, intended to send a far bloodier message!
But since that grizzly fate had been thankfully averted, he settled for a far more 'softer' approach.