Underarm's got nothing to do with this - that was a tactic used in cold blood, and thoroughly thought through. Okay, so McCullum knew what he was doing, but it was all over in a matter of seconds - as the WK, he had a job to do, and he did it.
Overthrows also have nothing to do with it - the onus is on the fielder to hit those stumps (or the wickie's gloves, at least).
There's nothing wrong with what NZ did, and Murali clearly had a moment of madness - it's definitely his fault, not McCullum's or the ump's. It doesn't mean that I have to like it, though.
Take backing up as an example - it'd be considered extremely bad form for a bowler to run a batsman out without giving him a warning first. In this case, the batsman is actually trying to gain an unfair advantage - surely he'd deserve to be run out regardless?
If NZ had decided not to appeal (I admit that this would be a difficult thing to "not do" on the spur of the moment though), I think they'd be lauded, and deservedly so - they'd be performing a gentlemanly act and that deserves praise.
So...
No, there's nothing wrong with it...but just because it's not wrong, doesn't make it right.