Given how many high-profile individuals Putin's regime killed in elaborate ways (falling out of windows, poison, explosives, beating, disappearing, or just old plain shooting on the street) I'd say chances of "vladlen" falling victim of some MOD/FSB/PMC conflict are higher than Ukrainian conspiracy. Despite being an utter pro-war nutjob he was quite critical of the MOD in particular.
Which would be even better... I mean Ukrainians could do a few attacks like that, but Russian power structures fighting each other could inflict much more damage on themselves. Fingers crossed.
I also think more likely that it was done by Russian power structures, rather than by Ukrainians. As you said, he was nutjob, but he wasn't even first tier propagandist like Solovyov, Skabeeva or Simonyan. Somehow I doubt that Ukrainians would make such operation which wouldn't have much effect, other than satisfaction. It's a bit similar story like killing Darya Dugina, but then it was obvious that target was her father, not she.
IMO, it's more likely that someone in Russia decided to remove him. And it brings message to other propagandists that they should think about what they're talking.
Anyway, nobody can feel safe now in Russia, even if they're so far way from frontline.