Disagree and criticize the statement made by someone about the negative effects of cannabis.
There are so many things wrong with what you said that it is impossible to detail them all here, but: 
a) there are very few problems caused by cannabis and very many problems caused by cannabis prohibition;
b) "pot people" will obey the law when the law obeys them and not before;
c) cannabis is not terrible, nor is it a gateway drug, as has been proven again and again and as I am sure you well know (because you're probably a troll);
d) murder for cannabis is a black market thing so that is an argument FOR re-legalization not for prohibition;
e) cannabis has been documented to provide numerous health benefits and it has many healing properties, which I am sure you also already know (because now I'm really sure you're a troll);