So he had Ax. Right? Maybe, AT.
I think you bet it right.

What could have happened? 1. Most likely, he folds and you take down the already decent sized pot. 2. In this case, since he is a maniac calling all-ins with Ax on a scary board, you take down a far far bigger pot. 3. He calls and turns over the other 8. Not likely enough to worry about. 4. He calls and turns over a higher pocket pair. In that case you spotted a leak in his game slow playing high PP, write it down and take advantage of it. (however since he is a known maniac, this is pretty much impossible).

Risk vs reward, when you think about these 4 possibilities, it is clear the expected reward for putting out that bet is higher than the risk of loss.