Killer B lost to Kisame because he didn’t know how to combat the shark sword and chose to infuse his blades with chakra when fighting Kisame, therefore draining all of his chakra, it was the worst possible strategy to try and defeat Kisame. In general, while Bee is the stronger shinobi, Kisame is his worst possible opponent matchup. Outside of when he goes into tailed beat mode, Bee likes to use a combination of kinjutsu and taijutsu, but both of which he cloaks his swords or himself in chakra, which is exactly what Kisame thrives on, he can just slowly absorb Bee’s chakra until he can’t fight anymore, which is exactly what happened when they fought. Bee underestimated Kisame, he should’ve gone to tailed beast form right away for the best chances of winning, if they were to fight again, using the information he learned last time, Bee would’ve made the fight a lot more competitive, and if he hit Kisame directly with a tailed beast bomb it would be over