Today, his fight against the inundating tides of water has claimed him, probably due to bacteria and other microorganisms getting in and rotting it from the inside out.

RIP our spikey friend, RIP.
Incidently, the sign behind the cactus advises only watering during specific months when the cactus is growing. We had warned the person who owns said cactus not to water it continuously to no avail. Now a poor innocent cactus is dead. It's always the ones with no voice who become the victims and I see there is a clear need for an advertising campaign:
NEVER OVER-WATER A CACTUS.
*Note, it is debatable if the cactus could at any point actually speak. I would say a definitive "maybe". The slight drooping of a spine could have been the silent cry for help that nobody noticed.