Tough AA river decision

[krammatrix]

Stacks: Hero has ~100, villain covers

Party $50 PL Hold'em
Preflop:

Dealt to HERO: Ac As

UTG(VILLAIN) calls .5, MP1 calls .5, MP2 calls .5, Button calls .5, SB calls completes, HERO raises to $3, UTG calls, all fold

Flop: Ks Qs 8d
HERO bets 4
VILLAIN raises to 8
HERO calls

TURN: 3c
VILLAIN bets 15
HERO calls 15

RIVER: 10c
VILLAIN bets 51
HERO ???

No real reads on villain, just sat down a few orbits ago, villain has the big stack at the table, and I haven't seen him do anything out of line. I was trying to think of what hands he could have, and I couldn't see QQ or KK, maybe 88 or QK, but I also thought maybe he semibluffed the flop with JT, and kept shooting. I didn't think he would bet so huge on the river with QK, so I basically figured 88 or JT, or a complete bluff. What do y'all think? I've always had trouble folding big hands, this one was really tough for me...