T. Lee thought he could put a stop to Chigurh, but as the story goes on, where he continues to fail, he realizes he can not.
The convo with the uncle, T. Lee learns that this is not some new evil he's facing (despite the violence, drugs, and money), but an evil that's always been there.
And if the realization that he might not be able to stop Chigurh didn't push him to the edge, the realization that he could not stop any of it is what makes him go off the deep end.
Instead of bravely facing it, which is the typical hollywood ending, T. Lee, a law man in a long line of law men, gives up.
that's my take.
one of the best movies i've ever seen. It moved it's way into my top 5 after one viewing.