Charlotte concluded that she could save Wilbur's life by playing a trick on Zuckerman because

... Mr. Zuckerman wouldn't want to kill a smart pig.

Explanation:

because of all the immigration, america became one big melting pot. the diversity of cultures from all over the world made america somewhat divided as well. people that immigrated stayed with their kind, and their traditions, while native born americans stayed with their kind and their traditions. normally, people born in american scorned the immigrants as well. the new religions also divided america. in summary, it took away from the american culture and identified america as the "melting pot."