Not at all, they are babies. Dominance is not an issue at this age. Why do you think Zim killed him? Its VERY rare that any rat would kill another. Poor Fievel could've had a seizure or a heart attack or a stroke which sadly can happen with badly bred rats, and with unknown genetics these awful things happen...I take it you got them from a petstore?Fievel I just found dead.
I am guessing Zim killed him. I just went into say hello to them, and the hammock was all ripped out of it's clips (well, two parts was, not the whole thing.) and Fievel was dead. Zim has been the dominate of the two I always noticed, but I didn't think it would turn to killing, as it was never in a worrisome matter, or so I thought, these are my first rats, so maybe I missed the warning signs.
Now, a question I have, would this suggest not to introduce another Rat?
I AM sorry for your loss, its terrible to lose one so young