Why did Boombata stay and help Conan against the horn guardians?

My guess was because they were vastly outnumbered by the guards so they kind of needed one another in that situation. Really the only competent fighters in the group were Conan, Bombata, and Zula. Knowing that they were outnumbered Bombata probably decided to help Conan fight off the guards just long enough for him and Jehnna to escape, if you remember eventually Bombata did try to close the wall on Conan (Bombata probably thought in closing the wall he would trap Conan thereby leaving him to be killed by the guards/wizard), of course that action was unsuccessful because Conan managed to make it to the other side of the wall at the last second. So Bombata did try to kill Conan he just did it in a indirect way.

Another reason could be that Bombata was just plain cocky and believed himself to be a better fighter than Conan so much so that he thought he would be able to kill Conan regardless of the time or place.If you remember at the start of the movie while Taramis and Bombata are in Jehnna's room (after Conan leaves), Bombata says "You don't need him my queen" to which Taramis replied "I don't need the captain of the guard to steal the key, that is the job for a thief". So you could tell from that exchange that Bombata was a bit cocky and fancied himself just as good a fighter if not better than Conan. Perhaps thats why Bombata didn't kill Conan while they were in the temple of the horn guardians, he figured he'd get Jehnna and the horn back to Queen Taramis in one piece first, and because he believed in his mind he was a better fighter than Conan he'd just take Conan out later.

I'm of the opinion Bombaata's back-and-forth on Conan is fairly simple: he had fallen for the big Cimmerian.

http://www.thecimmerian.com/a-return-to-the-daze-of-highly-insulting-a dventure/

(I'm only half joking.)

Dionen or whatever apeared in his true form and was killing his queen and her people he wanted this mistake fixed.