rats and rodents seem violent

well,my friend had bought a rat about 1-2 weeks ago.
   
i was with her when she bought the rat and the rat was healthy,playful and in nearly perfect condition. i went to her house yesterday to feed her rat and take care of it as instructed[she had left the house about two hours earlier] and the rat seemed violent.the rat was biting at me and was chewing through the cage,and it wasn't playful anymore and it was EXTREMELY difficult to try to take care of it. She bought two male gerbils three days ago,and i don't know if i should be concerned for her rat and the gerbils. the rat and gerbils are in two separate tanks,but can gerbils get violent like her rat did? if so,why do they get violent? why is the rat so violent?

I can't speak to the rat, as I've never had one, but I do know about gerbils. For the rat info: WEBSITE: http://www.afrma.org/ = American Fancy Rat & Mouse Association.
   
If the gerbils are litter mates, they may get along fine for the rest of their lives. Or not. Males in the same tank can have personal conflicts, & if they start hating each other, they do fight - usually to the death if they can. Keep an eye on the gerbils. If you see that either of them is injured in a way that looks like bites, you need to separate them. Any small animal cage will work (your friend can pay you for the cage; they're available at Petsmart for $20.00 or so).
   
To catch a gerbil who doesn't trust you, use a glass or plastic jar, corner the gerbil in a corner with the jar, slap your hand over the opening, & deposit the gerbil in the extra cage. The gerbil will need litter, a water dish or bottle, a food dish & a box to hide in & make a nest. Tissues & paper towels make good bedding.
   

I currently have 7 rats. Is your friend's a male or female? Females are much more active and have a good deal more energy so they will take more work than a male. There are several reasons it could become aggressive. In my experience with rats, moving them to a new home causes a lot of stress and that could be part of it. Also, being taken away from it's cage mates could be contributing. Rats are very social animals and almost always prefer one or more mates. While your friend may have to tolerate a few nips to the hand to gain it's trust the best bet is to work with the rat slowly, be very patient, and never be forceful with it. Talk to it with a calm light voice and it will get used to that voice as a "safe sound". A rat that has been caged with others and is then moved on it's own requires a lot of extra people attention. At least a couple hours a day is best.