Awaiting the arrival of a fugitive, a ruthless bounty hunter holds hostage the patrons of a sleazy bar but soon discovers the imminent arrival of another fugitive: a killer with advanced mental powers from an alternate reality.

Nice suggestion. My use of 'another' is ambiguous. I meant 'one more' and not 'instead of'.

"A bounty hunter holds bar patrons hostage in order to trap a fugitive but when a killer with superpowers from an alternate reality also shows up the hunter becomes the hunted".

Like that as a logline twist: the hunter becomes the hunted. (Maybe someone wants to collect a bounty on him.)

Like that as a logline twist: the hunter becomes the hunted. (Maybe someone wants to collect a bounty on him.)

"A bounty hunter holds a bar of patrons hostage in order to trap a fugitive but when another killer from an alternate dimension shows up instead- the hunter becomes the hunted."

I don't even know if that is true- but I got the feeling that is kinda what happens.
Removed the part about the mental powers because it felt like trying to squeeze too much into one line. I guess you could go with:

"A bounty hunter holds a bar of patrons hostage in order to trap a fugitive but when a super powered killer from an alternate dimension shows up instead- the hunter becomes the hunted."

Good questions.

The bounty hunter's aim is to hold the bar patrons incommunicado in order to trap the fugitive. He gets help from an unexpected source and the fugitive killer has a weakness which the bounty hunter discovers. I'm thinking of describing him as 'legendary' rather than 'relentless' to up his game.

Why is the bounty hunter holding people hostage? What does that act (not the location) have to with waiting for a fugitive? IOW: why must he hold the others hostage?

And if the fugitive who shows up has such extraordinary powers, isn't the fight (hence the plot) over with -- game, set match --in the fugitive's favor as soon as he arrives? What mojo has the bounty hunter got that enables him to have a fighting chance?