Big stack was active, clicked buttons sometimes and was a bit spazzy, but I think he has some brains (he was able to take pause and take advantage of him image opening strong hands UTG)
His stats 36/34/38 in 56 hands.

I run moderately active -- 21/17.

So I saw him 3betting a lot, but that`s 1st time he flatcalls.
I`ve seen him sending Bakin to rail (just after I was moved to this table) in a hand where he 3bet pre BB-CO ~30-40bb deep and b/3b-shoving AJ on K9x rainbow to river J vs Bakin`s T9. Weird hand, probably they had some history/dynamics.

PokerStars - $20+$2|2000/4000 NL - Holdem - 9 players
Hand converted by PokerTracker 3

Hero (MP): 94,200.00
MP+1: 303,573.00
LP: 33,768.00
CO: 29,752.00
BTN: 101,516.00
SB: 75,005.00
BB: 68,154.00
UTG: 57,802.00
UTG+1: 87,236.00

Hero posts ante 400.00, MP+1 posts ante 400.00, LP posts ante 400.00, CO posts ante 400.00, BTN posts ante 400.00, SB posts ante 400.00, BB posts ante 400.00, UTG posts ante 400.00, UTG+1 posts ante 400.00, SB posts SB 2,000.00, BB posts BB 4,000.00

Pre Flop: (9600.00) Hero has 9 9

fold, fold, Hero raises to 8,000.00, MP+1 calls 8,000.00, fold, fold, fold, fold, fold

Flop: (25600.00, 2 players) K Q 4
Hero bets 7,555.00, MP+1 calls 7,555.00

Turn: (40710.00, 2 players) 8
Hero checks, MP+1 bets 20,000.00, fold

Since my range hits that board hard I decided to rep nuts and watch how he would react. I expected him to raise pure draws vs small bet.

Later I thought it`s a bad card ott for my perceived range and since he just called I thought he often has some part of the board or even some slow-played AA+.
So I checked planning to decide and maybe to call small bets, he bombed turn and I folded.....

Absolutely fine. You can feel more comfortable with your decision given the stacks of the players behind who had yet to act when he called pre flop.

your hand is too weak to cbet this small and i'm not sure if i'd put more chips in in this spot

yeah do we need to even be c-betting this flop?

c/f flop imo

he still might have some junk, as Ive said he wasn't that disciplined.
Though even if has 75o we probably will finish at the same spot.
and one more consideration - if he checks back turn do you vbet low bricks otr?

I think you should bet ott. Op has a lot of draws here: fd and gsd, Qx and mb even some random stuff. Bet OTT folds a lot of his rng

Flop bet size is just awful. You might as well c/f if you bet that small. B/f bigger is my preffered choice.

Jackyl and Hyde?

Didn't really look at the flop size when I first read it

he wont flat junk all that often against an openraise from a 20bb stack and a gazillion reshoving stacks behind him. c/f flop.

yeah, just let this go. you will get folds very rarely here

i'm often c/f flop

flop bet size is really terrible, prob c/f this board

His flatting range pre is probably made up of mostly broadways, aj/a10 and some of the medium pairs. This board smacks that range. The only hands he will fold are like 66-1010, and he may even peel with those when you bet this small, just to take it away on later streets. Problem with betting flop (even betting bigger) is that he will peel with hands we beat (aj/a10/j10/etc) but we can't just barrel turns, or c/c. So he will be able to win the pot with his air every time, unless we bink a 9. Also, this is a prime spot for flatting huge given the reshove stacks behind so he could be slowplaying a monster as well. I just c/f this flop, especially against a villain who has heaps of chips.