Why would I cap?

[partygirluk]

I am sitting to the left (acting after) of a raging LAG who seems determined not to leave the table until he has lost all his money. He is down to less than 10BBs however. Table is 5 handed.

LAGtard bets into me on the river, and I raise. Typical players calls 2 cold, LAG 3 bets. LAG is bad, but he wouldn't do this unless he had me beat 100% of the time. You are also sure there is no way LAG will fold to your river cap. You have 3rd player on a weak made hand, likely better than LAGs, and think there is a chance he will fold for 2 more on the river. The pot is very big. Who can see why it might be best for me to cap here?