My take on the Russian pilots dumping fuel on the drone was they were hoping maybe for the engine to ingest the fuel and either catch fire or to damage the engine (it is a small turboprop engine). Failing to do either one, they start coming in close to try and hit the drone to damage it enough for it to crash.
The fuel dumping can be seen as malicious behavior, but not an overt attack on the drone. "Accidentally" hitting the drone can be claimed as just that, an accident while performing the malicious behavior, also without performing an overt aggressive act to destroy the drone. Had they hung back and launched a missile at it, their intent becomes obvious and indefensible as anything but an offensive move. Pretty smart on the Russians' part, I think. The declassified video released by the US from the drone's cameras makes it clear that whatever they were trying to do, it was intentional to damage or destroy the drone.