nothing other than; unmitigated, pure venom.
It wasn't as if what was said, although painful, was something that had to be said with the aim of helping in some way. No - it was MEANT to hurt - pure venom.
or, if the reply was a comment or opinion about another person, then it was pure hate, total disdain for the person, with the words being like undiluted venom.
It could be used in the same place, but it wouldn't replace it (like for like); it would modify the meaning. "Sheer", as David said, is unmitigated; "mere" is more depreciative. For example, "sheer stupidity" is nothing but stupidity but it's also absolutely/extremely stupid; "mere stupidity" is stupidity and nothing else. The two words occupy a similar semantic space, but they're not interchangeable.
Thanks everybody for such quick replies. I didn't expect to find so many posts. ;-)
Thus, depending on context sheer may convey two different meanings: 1) absolutely/extremely (used to emphasize something) and 2) nothing else but; while mere can only convey meaning 1).
Did I get the point?
you must have made a typo as mere can convey only meaning 2;-)