Glossary

dishonest, unprincipled low-life scavengers

This is a very insulting way to describe the British soldiers!  They were called 'lobsters' for two reasons: one is that lobsters are bright red when cooked, the same color as the soldiers' uniforms; the other is that lobsters are scavengers and, in colonial times, considered the lowest form of animal life. 

Scoundrel means a dishonest or unprincipled person.  Add them together and you see that the crowd was calling the soldiers 'dishonest, unprincipled, low-life scavengers.'