If you choose to kill Kematu, be ready for a horrific whirlwind combat- Kematu is quite tough despite wearing only Redguard Clothes, particularly with his pair of scimitar. The other seven or eight Alik’r are similarly garbed, though each only bears one scimitar, and they will viciously dogpile you. Again, a stand-back archery strategy works well, as there’s a chokepoint to the tunnel that leads back out of Swindler’s Den which you can use to clog them and take them down one by one. This is of particular use to characters in light armor or robes, who simply can’t otherwise withstand the assault. Heavily armored characters, however, will find that as the scimitar are essentially Iron Swords, they just won’t cause enough damage fast enough to bring you down. Heavy armor characters can feel free to slog in, or use a chokepoint- or even stand at the bottom of the watery ramp and knock the Alik’r aside as they come, since none of the Redguard warriors even have bows, much less facility with them.
If, on the other hand, you decide that Saadia is telling the truth, or that you want to catch Kematu in a situation where he doesn’t have half a dozen more warriors backing him up, you can talk to him and ask what he wants. He will tell you that he needs Saadia outside of Whiterun, since he and his men cannot re-enter the city, and asks that you convince her to meet you at the stables, where she can be caught to be brought back to Redguard.
You will then head back to the Bannered Mare, where you need to lie to Saadia and tell her that you weren’t able to defeat the Alik’r, who are coming for her. You’ve readied a horse, you say, and she needs to get on and ride. Saadia will start following you, so you need to head out of Whiterun and follow the cobbled road to the stables, where Kematu is waiting.
If you want to receive your reward from Kematu, or figure he is telling the truth, let him hit Saadia with a paralysis spell- he’ll give you your reward afterwards when you talk to him, and then you’ll never see any of them again.
If you instead believe Saadia, or would rather befriend and be paid by her, then attack and kill Kematu before he can paralyze Saadia. The ex-noble will complain about having been used as bait, but she will still give you her reward and return to the Bannered Mare, where she’ll have a slightly more personable attitude towards you.
Regardless of why you’re rewarded or from whom the reward comes, your reward for this quest is a solid 500 gold coins. Well, and whatever loot and skills you swiped and developed throughout the quest.