Rats are mindless, pycho, blood-thirsty killers who want nothing but destruction. They have no need to eat, sleep or any other activity beside killing every thing in sight including their own kind.
The Rats went under the procedure to be turned into a Blacksuit like the others but something went wrong. Either too much Nectar or their bodies weren't fit for the change could've caused them to become a Rat. Some Rats though have a small chance to remember their old lives just like a Blacksuit.