Something you might have missed

It is strongly hinted that sensei killed Grand Master and blamed it on a tragic hicking accident in which a hunter mistook him for a bird. But, here is something you might have missed: After killing Grand Master, Sensei might have cited the last rule, rule number 10: "If it works, use it".

After this event, Sensei added rule 11, which was clearly added later: "Guns are for the weak". He added this in order to prevent his own assassination at the hands of another aspiring student like him (infact remember, he even mentioned that Casey reminded him of himself). This makes Casey's justification "There are no rules" after shooting Sensei, all the more rewarding.