Give this a try, it's untested.
Code:
#include <amxmodx>
#include <engine>
#define PLUGIN "Headshot Knockback"
#define VERSION "1.0"
#define AUTHOR "stupok69"
#define KNOCKBACK_AMOUNT 600
public plugin_init()
{
register_plugin(PLUGIN, VERSION, AUTHOR)
register_event("DeathMsg", "event_DeathMsg", "a", "1>0", "3>0")
}
public event_DeathMsg()
{
new killer = read_data(1)
new victim = read_data(2)
new Float:aimvelocity[3]
VelocityByAim(killer, KNOCKBACK_AMOUNT, aimvelocity)
entity_set_vector(victim, EV_VEC_velocity, aimvelocity)
return PLUGIN_HANDLED
}