I think betting into dead pot to protect your hand is fine and accepted play especially in the mid stages of an MTT nowhere near the bubble which is a bit different. Getting the all in player out of the tournament is not the priority here, winning the pot is IMO.
It can be taken too far i get criticised in the chat box of an STT earlier for doing this, i think this etiquette has been misunderstood by many lesser poker players.
POKERSTARS GAME #6035663965: TOURNAMENT #30576055, $20+$2 HOLD'EM NO LIMIT - LEVEL VI (100/200) - 2006/08/24 - 15:33:05 (ET)
Table '30576055 1' 9-max Seat #7 is the button
Seat 1: lxa656 (2175 in chips)
Seat 3: GIJoe33 (485 in chips)
Seat 6: SofaKingDumb (3750 in chips)
Seat 7: silenthill76 (1405 in chips)
Seat 8: i jp A (4055 in chips)
Seat 9: Longy11 (1630 in chips)
i jp A: posts small blind 100
Longy11: posts big blind 200
*** HOLE CARDS ***
Dealt to Longy11 [

]
lxa656: folds
GIJoe33: raises 285 to 485 and is all-in
SofaKingDumb: calls 485
silenthill76: calls 485
i jp A: folds
Longy11: raises 1145 to 1630 and is all-in
SofaKingDumb: folds
silenthill76: folds
*** FLOP *** [

]
*** TURN *** [

] [

]
*** RIVER *** [

] [

]
*** SHOW DOWN ***
Longy11: shows [

] (a pair of Kings)
GIJoe33: shows [

As] (high card Ace)
Longy11 collected 2040 from pot
*** SUMMARY ***
Total pot 2040 | Rake 0
Board [

]
Seat 1: lxa656 folded before Flop (didn't bet)
Seat 3: GIJoe33 showed [

As] and lost with high card Ace
Seat 6: SofaKingDumb folded before Flop
Seat 7: silenthill76 (button) folded before Flop
Seat 8: i jp A (small blind) folded before Flop
Seat 9: Longy11 (big blind) showed [

] and won (2040) with a pair of Kings
The all in man commented that i should have flatcalled preflop as me isolating him, gave him chance to stay alive! I mean like i want to play kk 4 way here with my stack.