To explain that circumstances made them seem wicked, dirty, and lazy
In order for us to answer your question we would need more information in regards to where the ethnic groups are located, or the reason for distinguishing between ethnic groups.
These so called ethnic groups migrated from south America either driven by pirates or were pirates themselves venturing out to open seas. Finally some landed in the Carribean islands and settled there.
for ethnic reason
One reason is that many of its people identify more strongly with regional and ethnic groups than with the nation as a whole. Most of the population are of British and French ancestry.
The most likely reason he described them that way was to make it appear that they would be easy to defeat.
well there might be lots of reason why people having stereotype views towards other ethnic groups, examples are cultural misconception, conflict between two ethnic groups historically, etc. For the Japanese case, Chinese and Korean disliked them because Japanese historically had invaded these two countries and committed lots of war crimes. Some others might have other reason, mainly cultural misconception and their (majority) difficulties on communicating in other languages, I assume?
ethnic conflict
The most likely reason he described them that way was to make it appear that they would be easy to defeat.
One reason was jealousy. For example, if the Serbians got their own country, then why couldn't the Irish? Because of the formation of one country, other ethnic groups got jealous and wanted the same. Nationalism was widespread in Europe, starting from the late 1800s.
The most likely reason he described them that way was to make it appear that they would be easy to defeat.
Groups are formed for a particular reason of achieving a goal.
PERSECUTION : to hound, oppress, or harass a person or group without just reason (originally used for killing, exiling, or subjugating religious or ethnic groups, e.g. Christians in the Roman Empire, Jews under Naziism)