The collectors wish to exterminate any race on Omega, with exception for humans - for testing. They've make a deal with the Blood Pack mercenaries to make use of the vorcha to get into Omega and set out the plague through the ventilation without being noticed.

Even if most vorcha are known for their blood lust and hunger for battles, some have a bit more intelligence than others. While the collectors decided to finish the deal with the vorcha, three vorcha, under the leadership of Vorka, decide to refuse to take part in this deal and attack the collectors that are present head on. Since the collectors had disabled all flamethrowers, the vorcha can't use these.
Vorka has heard about a salarian doctor who is also good with weapons who came to live and work on Omega. The salarian has his own clinic and will stop at nothing to protect his own people. Vorka may not know fear, but he knows who to be careful around. This doctor is noteably dangerous, more so than the collectors who try getting them to work to spread the plague into the Omega slums.

Running around the battlefield, the vorcha seem hard to catch and impossible to kill for the collectors, so they're sending in their most powerful troops to take these three out. Whether it be at a distance or at close range, the vorcha keep making it increasingly difficult for the collectors.

They're trying to reach the Blood Pack leader of their team during the mission, as they need a way out of there. When they reach him, he tells them to keep the collectors busy while he is trying to get a ride to get them out.