Even a low-level character should not be seriously threatened by a giant rat...that's like your soldier coming back from a tour in war and getting killed by a corgi
Actually, according to the d20 srd, a dire rat grows to about 4 feet long and weighs over 50 pounds. And that's listed as a smal l creature. In the D&D Next packet, it's listed as a medium sized creature. So it's really more like you getting killed by an average sized pit bull (30-60 pounds). One thing that I think makes D&D's fragile characters so odd to so people is that we're generally very well protected against the dangers nature poses to us on a day to day basis. Nature is vicious and will kill you without remorse given the opportunity, and there are a number of small things, smaller than corgis, which will lay down even the strongest of men without modern medical attention.