AA Getting Donked into by Aggro opp

Villian is 18/12 and aggressive postflop over 80 hands. I just called the flop because of him being so aggressive and also because I had the A of spades, which takes away all Axss flushdraws. Should I have just raised? The turn I called planning on calling all river bets. When he checks I throw in a bet I think Kx hands will call. Then, wtf, should I just fold now?

Full Tilt No-Limit Hold'em, $0.50 BB (7 handed) - Full-Tilt Converter Tool from FlopTurnRiver.com

UTG ($87.35)
MP1 ($62.85)
MP2 ($9.25)
CO ($60.80)
Hero (Button) ($51)
SB ($50.25)
BB ($52.80)

Preflop: Hero is Button with Ahttp://www.flopturnriver.com/phpBB2/...iles/heart.gif, Ahttp://www.flopturnriver.com/phpBB2/...iles/spade.gif
UTG calls $0.50, 3 folds, Hero bets $2.50, 2 folds, UTG calls $2

Flop: ($5.75) Khttp://www.flopturnriver.com/phpBB2/...es/diamond.gif, 5http://www.flopturnriver.com/phpBB2/...iles/spade.gif, 8http://www.flopturnriver.com/phpBB2/...iles/spade.gif (2 players)
UTG bets $5, Hero calls $5

Turn: ($15.75) 6http://www.flopturnriver.com/phpBB2/...iles/spade.gif (2 players)
UTG bets $8, Hero calls $8

River: ($31.75) 3http://www.flopturnriver.com/phpBB2/...es/diamond.gif (2 players)
UTG checks, Hero bets $19.50, UTG raises to $39, Hero calls $16 (All-In)

Total pot: $102.75 | Rake: $3

very strange line from utg

As illfavor says, spenda and others also say... spoon especially says...

And as an experiment for myself... ranging this player might be difficult based on this line, but limping UTG as an 18/12, I'm giving him a range of 22+, AJs+, KTs+, AQo+. But also, I think we can rule out AA - QQ, as UTG would probably bet, not call those to open. Unless you're seeing something 'trappy' in his play.

So I'm thinking he may be on a KQs? Maybe AK with Ks, his line is really funky on the turn/river. To lead flop with nearly pot bet... then slow way down on the turn? Unless the spade scared him, but then to over-shove? I'm really thinking this line indicates Kxs to me... he really looks like he wants you to take the lead so he can get it in.

But I might be way off. Comments appreciated on all aspects of my casual player analysis.

I don't like calling the flop. I raise for value. He will call with the Kings in his range, plus charge him for the draws, if he is that aggro he might even reraise all in with a KJ,KQ,AK or flush draw.
Don't call get the money in on the flop.
On the river opponent either is turning his hand into a bluff, has pure air, or had you all the way.
If he had you beat all the way, then it's a stupid line since you can check back and he looses so much value.
I still think we are ahead so I call the river.