It's not so much that combat "is extremely lethal at low levels" as combat "has a chance of being lethal at low levels"... This is true of any system with detailed critical and fumble rules, and not just for "low levels" though RM/MERP do have a reputation for killing low level characters unexpectedly (often by accident). Compare with Avalon Hill/Chaosium Runequest which killed characters of any power unexpectedly with lucky/unlucky crits and fumbles (my first character at Uni got all the way through a major questline the DM had written, then died to a speed-bump random encounter).
Also for new players if the DM is experienced they may well fudge some of the dice rolls (such as re-rolling any instant kill criticals). Fumbles are not so likely to be instant kills (iirc the worst one is "severe groin strain, -75 to all actions, opponent helpless 2 rounds with laughter" which if you have allies and your opponent doesn't will kill the opponent!)
Yes you don't have that many hit points, but most RM fights end of a kill critical anyway, not from hit point loss.
Note, I may be wrong about the fumbles - that's just the worst one I remember, probably because it is so funny.