When Bowen kills Draco in the end, Bowen fulfills the two vows he made in one move—to serve Draco whenever he needs him and kill Draco wherever the dragon goes.
Early in the movie, Bowen and Draco engage in a Mexican Stand Off, where Bowen stands with his sword upwards inside Draco's open mouth—no matter who kills who first, the other one will die too. So, why didn't Draco Take a Third Option and roast Bowen alive to get rid of him unharmed? Throughout the movie, Draco is always seen breathing fire through his nostrils, meaning he didn't toast Bowen, possibly because he can't breathe fire through his mouth.
It could also be because Draco never wanted to kill Bowen in the first place, as he explains afterward, although he may have been perfectly capable of doing so. Draco seems to be holding back throughout the fight and trying only to disarm Bowen rather than kill him. Draco doesn't aim his fireballs directly at Bowen, uses his tail for wide swings instead of his claws, and never goes for the killing blow.
If you observe, it’s not until Bowen reveals he killed the female dragon that Draco finally loses his cool, stops holding back, and grabs Bowen in his jaws to go for the kill, only to be caught off guard with the “standoff” which gives him time to calm down.
Draco looks at the rope on the ground connected to his foot twice while taunting Bowen as the man unknowingly approaches it before disabling Bowen by pulling it taut. The novelization and an early script explain that Draco planned the whole thing. He insults Bowen to annoy and make him move until he straddles the rope where Draco wants him.
In a way, Draco ultimately emerges as the decisive winner since he pins Bowen, who loses his sword while falling from Draco's mouth and doesn't have his shield on hand to help him. Bowen does fight Draco with both weapons in early scripts and the novel but loses his shield when Draco trips him up.
Einon intends to make Kara his queen, but how? The movie implies that Einon rapes Kara, but he succeeds based on a deleted scene, the novelization, and early scripts. The book describes how Einon hopes to own Kara by stripping Kara of her power and purging his fear of her by proving he's the more powerful man, as his father did to his mother. Like "breaking a wild horse," Einon wants to kill Bowen, stick his head on a pike, and show Kara to break her spirit and make her submissive.
Draco mentions having to live with Einon's every dark thought and impulse since sharing his heart. It must have negatively affected Draco's psyche somewhat, so what would've happened if Bowen and his army imprisoned Einon instead of killing Draco? Draco says a dragon must be careful when sharing their hearts with people as giving half to the wrong kind of person can doom them in the afterlife—what if a dragon remains bonded to an evil human for too long? Would the human's evil corrupt the dragon if its will isn't strong enough to resist? Would the dragon suffer fugue states where they'd act out of character and dangerous before soon devolving into a wild beast unable to talk or be reasoned with? If Einon were jailed instead of killed, his malice would only grow, further damaging Draco's mind and heart and dooming his chances of going to heaven. After living with Einon's evil for 12 years, maybe it's fortunate that Bowen re-enters Draco's life when he does.
This may explain why Draco didn't just wait for Einon to die of natural causes. While the later prequel films like Dragonheart: Battle for the Heartfire establish that a human bonded to a dragon can still die of old age, it's easy to see why Draco didn't want to endure living tied to Einon that long, especially considering that Einon was young and healthy with no way to know what would happen if Einon's captors tried to kill him by starving him to death or poisoning him for example.
