The dealer dealt all the cards to 10 players, 6 players are now in the hand, so he flopped 3 cards on the table and all 6 players checked. So as he was going to do the turn card he messed up the top 5 or 6 cards and he was not sure they went into the original order so he ruled the hand dead, Is this the right decision?
No, the hand is not dead.
Looking at Robert's Rules of poker here the following is included in rules for misdeals.
In button games, action is considered to occur when two players after the blinds have acted on their hands. Once action occurs, a misdeal can no longer be declared
As action has occurred (players calling pre flop) then this rules that the hand can not be declared dead.
I'm confused as to how the top few cards would get mixed up but as long as they haven't got mixed with the muck (previously folded cards, or discard pile to boardgamers!) then the cards coming off the deck will still be random and continuing is possible.