For a supposed badass mercenary, Marc never truly wins.
He fails to keep Steven unaware of himself and Khonshu. He fails in keeping the scarab away from Harrow. Khonshu leads them to Cairo, and Khonshu summons the other gods after Marc fails to gather information from Harrow's goons. Hathor's avatar gives Marc the clue about Senfu's sarcophagus, while Layla gets him there and Steven finally deciphers the star chart. Marc then doesn't front until Layla confronts him about her father, and they get shot a few moments later.
All in all, not a great track record.
But Marc's real fight was never to stop Harrow. Marc's real fight was to keep moving and not give up.
While it finally becomes clear what Marc has been dealing with in episode five, he hints at it far earlier. Marc promises Steven that he will disappear after the deal with Khonshu is over in the second episode, and reinforces this in the fourth. He also tells Steven that it has been hard to even be a fly on the wall. In addition, Marc says he should have died when Layla confronts him about her farther, and he says something similar while they are down in the room with all the people he has killed.
Marc's true strength lies in the fact that despite everything, he still puts one foot in front of the other and keeps fighting. Marc was presented the opportunity to stop and be rid of Khonshu three times, each time with the cost of Harrow releasing Ammit. The first time during the trial, the second time in Mr. Great's tomb, and the final time in the Field of Reeds. But all three times he chose to go on. Despite Khonshu's abuse, despite that he was still reeling from his mother's death, and despite having to leave eternal paradise behind, Marc chose to fight.
And he did it for Layla and Steven.
If that isn't the sign of a hero, I don't know what else is.