Runner runner flush...did i underbet or was it inevitable?

[LL2318]

BUTT is lucky lucky often preflop raise passive pf so i call preflop

$1/$2 Blinds No Limit Hold'em - *** 08 31 15:06:24 2006
Seat 5 is the button
Total number of players : 5
Seat 1: BUTT ( $352.6 )
Seat 5: stupid ( $96.15 )
Seat 7: HERO ( $143.1 )
Seat 8: stupid ( $243 )
Seat 9: stupid ( $537.11 )
HERO posts small blind [$1].
bocean posts big blind [$2].
** Dealing down cards **
Dealt to HERO [ 8d 8s ]
stupid calls [$2].
BUTT raises [$8].
stupider folds.
HERO calls [$7].
stupiderer folds
stupiderer folds.
** Dealing Flop ** [ 8h, 6s, Kc ]
HERO bets [$7].
BUTT raises [$20].
HERO calls [$13].
** Dealing Turn ** [ 2s ]
HERO checks.
BUTT bets [$28].
HERO raises [$56].
BUTT calls [$28].
** Dealing River ** [ 4s ]
ll2318 bets [$59.1].
BUTT calls [$59.1].
** Summary **
Caorlie shows [ 10s Qs ].
HERO shows [ 8d 8s ].
HERO shows [ 8d 8s ].
BUTT shows [ 10s Qs ].
BUTT collected [$286.2].

Villian is donk passive donk so i call his preflop raise
Flop I try to induce a raise with a weak bet, succeed yay
Check-Raise turn, guess here i underbet? i wanted to keep him in...
River All In