Why the Bartender? spoiler

That, perhaps. On the other hand, Books was a notorious gunman who had just killed three people in his bar, including the establishment's own faro dealer. The bartender had been in the back room during the shooting; so, theoretically, it's conceivable that he thought that Books had murdered them and, possibly, might have gone on to murder others - including, had Books turned on his way out, perhaps the bartender, himself. The viewer knows better, of course, but that could, theoretically, have been the bartender's thinking.

YOUR COMMENT JUST GAVE AWAY THE ENDING OF THE MOVIE !!! YOU SHOULD EDIT THE TITLE TO SAY
"*Spoiler Alert* Question about the bartenders actions"

Geeeeze ...

[deleted]

Perhaps it was bragging rights. Perhaps he was friends with the people Books just killed. Perhaps Books did something that offended the bartender way back when.

The audience doesn't know. The idea is that the bartender is a nobody, a nameless man who got the jump on the iconic gunslinger. The death corresponds with Books' statement earlier, that it's usually the amateur you have to worry about.

Maybe bragging rights but really I don't know the era but the guy that shot billie the kid was called a coward.

I recall hearing that it was John Wayne's demand to change the script so his character would be killed by the bartender.

Earlier in the film Books said "There's always some six-fingered bastard that couldn't hit a cow in the tit with a tin cup. That's the one who usually does you in."

So it was kind of foreshadowing someone other than the three gunmen would kill him.