Villain is bad reg 23/17/7 .
Blocking Kd is what confuses me in this instance, but seems the right fold as he shouldn't be too concerned about flushes esp when i check this flop?
Can also be bluffing spades but would have to be QJss/KJss/JTss/QTss and is he really 3 betting these pre and then running a bluff on paired aces?

PokerStars - $0.10 NL FAST (6 max) - Holdem - 6 players
Hand converted by PokerTracker 4

BTN: 170.9 BB (VPIP: 22.64, PFR: 16.92, 3Bet Preflop: 6.77, Hands: 461)
SB: 174.5 BB (VPIP: 20.00, PFR: 0.00, 3Bet Preflop: 0.00, Hands: 6)
BB: 83.7 BB (VPIP: 23.81, PFR: 19.05, 3Bet Preflop: 0.00, Hands: 24)
Hero (UTG): 120.5 BB
MP: 115.7 BB (VPIP: 18.32, PFR: 13.86, 3Bet Preflop: 8.43, Hands: 207)
CO: 105.3 BB

SB posts SB 0.5 BB, BB posts BB 1 BB

Pre Flop: (pot: 1.5 BB) Hero has K K

Hero raises to 3 BB, fold, fold, BTN raises to 9 BB, fold, fold, Hero raises to 21 BB, BTN calls 12 BB

Flop: (43.5 BB, 2 players) T 5 A
Hero checks, BTN checks

Turn: (43.5 BB, 2 players) A
Hero checks, BTN bets 22.8 BB, Hero calls 22.8 BB

River: (89.1 BB, 2 players) 3
Hero checks, BTN bets 127.1 BB and is all-in, fold

BTN wins 85.1 BB

How do you know he is a bad reg?

Do you have any Ax hands OTR? This will determine if you theoretically have to fold or not.

As an explo move we can fold this

Wp, the only street that's open to debate imo is the turn where a thin value bet could be justified. I would 4 bet larger pre but in practice I think I play the rest of the hand the same.

Seems fine to me. I think I would just call with my worst Ax's.

How many Ax combos does villain 3bet preflop against UTG? most players at NL10 only 3bet JJ+ AK against UT, which we block. He could also be 3betting A5s, but I don't think he would call a 4 bet with that hand (unless he is actually a very bad reg). He could also 3bet some suited connectors, but those wouldn't call a 4bet. So there are 4 combos of AK left in the deck and one combo of aces (which he might have slowplayed). So you're losing to 5 combos of hands which he could possibly have at that spot after a 4bet.
Could he turn a hand like jacks into a bluff when you checked twice. Also would he shove an ace on the river, because your line looks so weak he should think that u are weak.
I'm an exploitative player so this could sound wrong to a GTO player, but thinking that it's NL10 I would call knowing some people could punt their entire stack with QQ in that spot. Also if you call and he has a hand like A5s or AQs you are able to exploit this in the future.