Why didn't Blondie leave Tuco hanging

Cuz he's "The Good" 

Besides, he had to expect some ill-will after he made Tuco walk 70 miles through the desert back to town. 70 miles! 

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Because they were even. First Blondie screwed Tuco by leaving him tied up in the desert; Tuco got back at him by making Blondie march through the desert. Earlier, Tuco made Blondie get ready to hang himself in the inn, so Blondie got back at him for that by making Tuco stand with his head in the noose in the cemetery. That evened the score. Blondie is "the good" because he has a sort of honor code, a sense of fair play, even though he could be an assh-le too. After what Blondie had done to Tuco earlier, he had to expect that Tuco would get back at him for it. As Tuco had said, "whoever double-crosses me and leaves me alive, he understands nothing about Tuco!" The march through the desert was Tuco's revenge and Blondie knew he had it coming. He had no grounds to kill Tuco at the end of the movie.

The people, and the people alone, are the motive force in the making of history.
-Mao Zedong

I think there was certain degree of friendship between the two.
At least a mutual understanding and respect..?



Thumbs Up, Thumbs Down and a Wagging Finger of Shame

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Can you really blame Tuco for doing that? That was just revenge for Blondie screwing him over and leaving him to die in the desert earlier in the movie. Blondie deserved every bit of Tuco's retribution.

Well - Blondie and Tuco at the end of the film are not Blondie and Tuco at the beginning. The characters change - just like real persons do. They've done a lot of riding together and have become (kind of) friends.
But not as close friends as for instance Juan and John in 'Dynamite' -- in that case you could almost talk of them being in love (when John blows himself and the train up Juan mumbles What will become of me now