To be fair, if I was DMing this dragon in an actual campaign, I would have most definitely had it retreat once it dropped so low in hit points. Dragons are not going to willingly fight to the death in 99% of cases. Other than NOT running away when badly wounded, I don't feel like I ran it THAT poorly. It successfully avoided melee range with the Fighter and Barb, and it did focus on the most dangerous character, the Rogue almost killing it.

The point of this was could our party survive an encounter with an adult red dragon and then could they actually kill one. I was very skeptical, but responses were mostly favorable in the party's favor due to action economy. I was surprised our party was able to kill it at all, and we probably wouldn't have had it been able to recharge it's breath weapon.

If I was to run this again as another white room fight to the death, I would maybe have it use it's fire breath, and then retreat until it recharged each time. I also didn't feel right using it's Wing Attack with no enemies around simply to get the extra movement. Maybe I'm wrong, but that doesn't make sense.