the fault does lie with the batsmen, but as a batsmen if i were an inch outside the crease by mistake i would be fuming. i also believe that its almost impossible for a bowler to fully start their action and mankad the batsmen without pausing - which in itself would it not make it a dead ball?
There is no requirement for the bowler to start their action. The bowler can execute a run out as soon as the ball becomes live. It only becomes a dead ball if the attempt is unsuccessful.
41.16 Non-striker leaving his/her ground early
41.16.1 If the non-striker is out of his/her ground at any time from the moment the ball
comes into play until the instant when the bowler would normally have been expected to
release the ball, the non-striker is liable to be Run out. In these circumstances, the nonstriker
will be out Run out if he/she is out of his/her ground when his/her wicket is put down
by the bowler throwing the ball at the stumps or by the bowler’s hand holding the ball,
whether or not the ball is subsequently delivered.
If you're an inch out you're unlikely to be given out unless there is a replay available, which rules that out of most games. If you're a foot out, it's easy for the ump to see, and you deserve to be given out.
It's the batter's job to make sure they are in their ground whenever the ball is live, and if they are not, it's the fielding side's job to run them out or stump them. If you get out this way, it's because you made a mistake - exactly the same as being out caught, bowled LBW etc.
The more I think about it, the less problem I have with it.