Surely Hanlon's "don't assume malice" falls down a bit in state sponsored assassinations?

Yes, but when the question is whether they’re deliberately giving the impression they’re incompetent to add insult to injury, I’m inclined to assume they’re just plain incompetent.

But they do that a lot in other contexts, like using novichok agents or traceable radioisotopes for state assassinations, then 'denying' it was them.

i mean the "getting caught out of own incompetence" part of course. a recent example was Navalny's call to his poisoner from the FSB. This was obviously NOT an intended leak

This exactly how it is. The way they covered up murders during protests is very descriptive of their ability to do such things.

https://en.wikipedia.org/wiki/Death_of_Alexander_Taraikovsky