What or who wanted Mozgovoy dead? Who or what killed Alexey Mozgovoy?

The deliberate political assassination of Alexey Mozgovoy, so similar to the assassination of Bednov "Batman," in which Plotnitsky and his direct associates happened to be openly involved, was aimed at removing and suppressing what Mozgovoy represented. Thus, establishing what Mozgovoy represented does not only allow us to understand what "threat" he was posing to the master-minders behind the liquidation commando, but also the motive and probable identity of those who ordered  the assassination.

1. First and foremost, Mozgovoy was the one Novorossiya leader who openly declared and insisted that oligarchy (regardless of its nationality) is his and Novorossiya's main enemy. See, for example, here.
2. Second (in importance), Mozgovoy was the most outspoken leader and active commander who stood for the idea of Novorossiya and thus also for Russia's own rebirth to her own dignity and integrity.
3. This second principle also translated into Mozgovoy's third principled stance. Mozgovoy not only knew (as Putin, Lavrov, Kofman, Tzarev and also Russia's enemies knew and know so well) that Minsk and Novorossiya are mutually exclusive, but, unlike those mentioned together in the parentheses, Mozgovoy committed himself to Novorossiya, while the others chose Minsk over Novorossiya.
4. Mozgovoy was no one's pawn, he was his own man, and, also importantly, he was a genuine leader who emerged from the people themselves (he was not oligarchs' or elite's appointee). He also had a genuine charisma and had a genuine wide popular support, which extended not only beyond Donbass, but also beyond Russia and inspired and motivated many people around the world, thus offering a focus of a genuinely new consensus and movement.
5. He was Strelkov's friend and comrade.

I think that these basic points are quite sufficient. All these characteristics represented a perceived vital threat to someone's basic interests. What Mozgovoy had by virtue of his character and genius, those who ordered his death greatly feared and did not possess. In other words, the basic characteristics of the powers behind the assassination, which they ordered, are:

1. oligarchic or pro-oligarchic character and class interest;
2. fear or even rejection of Novorossiya as an emancipatory project based on renaissance of people's power and dignity;
3. commitment to Minsk and choice of Minsk over/against Novorossiya and over people's anti-oligarchic and antifascist liberation;
4. existential fear for one's popularity and ability to control and manipulate together with a genuine anti-revolutionary animus;
5. someone who is afraid to even mention Strelkov's name and who very likely hates him for everything Strelkov did and what he represents.

Long live Novorossiya!