The gemara says that in gil'ad there were many murderers, therefore the amount of cities of refuge for the ratio of the population, was greater. Many meforshim raise the question that the city of refuge is for accidental killing and shouldn't be related to the number of cold blooded murderers?
1. Tosafos answers based on the "mashal ha'kadmonim" that the more intentional killers, the more it was necessary for Hashem to arrange for their accidental death, thereby increasing the number of accidental killings. Therefore, the increased number of murderers demands an increase in the numbers of cities of refuge.
2. Maharsha explains based on R. Yossi Bar Yehuda in the Mishna - immediately following the murder whether intentional or accidental, the murderer would always run to the city of refuge. It was necessary to increase the number of cities for the intentional killers to run right after they killed.
3. Ramban in chumash (quoted in margin) writes that the more intentional killers there are, the more they make the killing seem as a mere accident. Therefore, as the killers increased, it became necessary to increase the numbers of cities of refuge.
4. I would like to suggest another approach, based on the Beis Halevi in Parshas Noach. The more corrupt people become, the more corrupt the world becomes. Laxity in abstaining from aveiros has an influence on society being more liberal and lax with aveiros. Therefore, the more murderers their are who kill intentionally, the less value their is for human life. It would follow that people would be less careful and more negligent with lives of others, leading to more accidental killings.
There seems to be a theological problem with all these approaches. The mitzvah to set up cities of refuge was given before the inhabitance of the east side of the Jordan. By their being a mitzva to set up multiple cities of refuge, because there will in the future be more murders, it became a self fulfilling prophesy and borders on the killings being predestined. But, based on the Rambam's classical approach that a decree on the public doesn't take away from the bechira chafshis (free choice) of the individual, it all works out.