rat killed rat???

Question
Hi there,
My boyfriend and I got rats for christmas from petsmart and when I woke up this morning I found my rat huddled shaking in a corner with my boyfriends rat dead in their nest with his stomach torn open.

Not that I think mine killed the other one, after all my surviving rat was about half the size of my boyfriends, But they did rough and tumble every so often.
What should I do about my remaining rat? I've been giving him a lot of attention and cuddles, he's never bitten me but he is significantly less affectionate. he's still young would he handle a new play mate? I don't want the same thing happening again.

Answer
he should be fine with another play mate, depending on his age, but being he did kill the other rat, or at least fed on him.. he could be more aggressive towards a new cagemate, he will do fine by his self, as long as your able to play with him for 20-30mins a day he will be fine.