There is no doubt that Pat Tillman was a hero. If forsaking a multi-million dollar contract in order to put himself in harm's way is not selfless heroism, I have no idea what deed might fit that definition.
Based on everything I have read, his death was caused by a number of gross errors by those in command. Since I led two advisory teams in Vietnam, I believe that I can comment authoritatively concerning the decisions by his commander, pending additional information, which has not yet been communicated to us:
1. One of the unit's vehicles broke down. The commander decided to leave half of his team with the vehicle in a small Afghan village, while the remainder were to return to their base camp. Unfortunately, the entire team was too small to defend against a substantial attack, and splitting the team made that situation even worse.
2. The team in the village was able to find a vehicle to tow their disabled vehicle, and decided to follow the first unit, apparently without notifying that unit of it's course of action.
3. The first half of the team heard them coming, assumed they were an enemy unit, and prepared an ambush, firing on their comrades when they came in range.
4. The second half of the team thought they were being attacked by enemy soldiers, and decided to return fire. Among them was an indigenous guide or interpreter, who had been allowed to carry an AK-47. That weapon has a distinctive sound, which is substantially different from our M-16's. In addition, the weapon probably fired green tracers - our M-16 tracers are red.
5. The first half of the team, watching green tracers from the second unit, was now certain that they were facing an enemy unit, and did what it could to wipe them out. In Vietnam, I carried a pocket size code book, which could be used to encode my transmissions, and I suspect that the Special Forces commander had one as well. When in doubt as to the automatic weapons fire coming in their directions, the commander of each of the two elements should have taken a few moments to encode a message to the other unit, but that would have been difficult by the rapidly diminishing light of the setting sun, and the use of a flashlight would have revealed their position to enemy forces in the vicinity.