PreThink is called hundreds of times per second, a bad place to send uncontrolled messages from.
Use "Health" event with a task instead.
PHP Code:
random_num(0, 1000) <= panic_chance
People would expect a chance CVAR to be in "percents", at least specify 2 is actually 0.2%
You only need to cache msgid once in plugin_init, message ids don't change during game.
Feature suggestion: Add a way to restore the player back to normal (for example, when they score a kill). Otherwise, it becomes a certain death without a way to gain hp.