Think a bit, whats the point to "send packet" to player object which is null?
Since its L2PcInstance, you don't have to use object of player, simply use:
sendPacket(new CreatureSay(0, Say2.PARTY, "Server", "[" + player.getLevel() + "]" + player.getName() + " vs " + "[" + target.getLevel() + "]" + target.getName()));
instead of:
player.sendPacket(new CreatureSay(0, Say2.PARTY, "Server", "[" + player.getLevel() + "]" + player.getName() + " vs " + "[" + target.getLevel() + "]" + target.getName()));
And about target object, it was just example, do not create new L2PcInstance object called target, simply you have to refer to this player which was killed by our player (ye ye, It may be confusing now)