couldn't muck A's

[Reef]

an OK MP raises, 3 callers, I'm in the BB with A's. I 3 bet and original raiser caps. Flop is KJx. I bet, MP calls, 2 callers behind. Turn is a blank, I bet out, he raises. Now I tell myself he has K's. 1 caller behind MP and I call. I check call the river since it was so big and sure enough he has K's.

Now, I knew he had them but didn't muck. Is this kind of thinking smart to where I should've trusted my instinct/thought process and folded? Or weak tight?

[Fudomyo]

Unless you really know your opponent, you can't muck there. He could have AK easily, or QQ if he's really aggressive.

I can think of plenty of times where I was in the same shoes as you and split the pot.

[axioma]

"Now, I knew he had them but didn't muck"

you did not KNOW he had kings. you might have strongly suspected he had them. as you rightly say, the pot was big enough to justify calling for those times your suspicion turns out to be false.