Bet/Fold on River or Bet/Call?

Hero is UTG with K [img]/images/graemlins/heart.gif[/img] K [img]/images/graemlins/club.gif[/img]

No reads on any players. Table seems pretty loose passive in 2 orbits.

Full Tilt: 2/4 table, 9 handed

Preflop: Hero raises, UTG+1 calls, 3 folds, MP calls, CO calls, Button calls, SB calls, BB calls

Flop: (14 SB) 7 [img]/images/graemlins/club.gif[/img] 5 [img]/images/graemlins/heart.gif[/img] 2 [img]/images/graemlins/heart.gif[/img]

SB checks, BB checks, Hero bets, MP calls, CO raises, 3 folds, Hero 3-bets, MP folds, CO calls

Turn: (14 BB) 8 [img]/images/graemlins/club.gif[/img]

Hero bets, CO calls

River: (16 BB) 8 [img]/images/graemlins/heart.gif[/img]

Hero bets, CO raises, Hero calls out of spite

CO shows A [img]/images/graemlins/club.gif[/img] 8 [img]/images/graemlins/diamond.gif[/img]

Should this have been a bet/fold, or was I ok to call here?