While I was in residual proximity of the whole thing (I ended up in the same company but different platoon, and I knew/worked with people in Tillman's platoon), I wasn't involved in it as I showed up about three years after the fact.
There definitely was no motivation to murder him, I'm convinced of it. Armed conflict inherently breeds a general cynicism, and lack of clear goals and guidance from leadership only exacerbates that. We all said out loud, "this is fucking bullshit," more than once when we would give full-throated efforts only to be frustrated by sideways shit like innocents or friends being maimed/killed, no clear mission or point to the killing and violence in general, a commander who seemed more eager to give a teary-eyed killed in action speech over a dead Ranger than to actually lay the hammer on insurgents, etc. All that to say that Tillman's misgivings about the war were especially normal. The real conspiracy was in the outright lying done about the whole thing, and in institutional leaderships' efforts to cover everything up.
Tillman was killed because of miscoordination/incompetence. The miscoordination at the macro level was from Hodne as I mentioned earlier, in his attempted orchestration of troop movement that introduced too much chaos. The incompetence came from a few variables of the people on the ground that I won't get too much into, except to say that it wasn't out of nefarious intent but instead from confusion and bad training. The SAW gunner who pumped rounds into Tillman had a reputation after he left of a bumbling, incompetent rock farmer. Note that I never met him personally, it was just his reputation around the company.
His death wasn't taboo internally. His named was invoked pretty regularly when it came to target selection and deciding if you had enough awareness of the assault forces' positions to pull the trigger into a room or general area; you never wanted to accidentally kill your buddy. A new Ranger would be liable to get smoked[1] until fluid leaked out of his ears because he "could kill another Ranger like Tillman, you fucking moron." Externally, we were ordered to divert all questions/comments about Tillman to the SOCOM Public Affairs Office.
[1]: "Getting smoked" in the Army (military?) is a euphemism for punishment through strenuous exercise/tasks, usually accompanied with humiliation and general hazing