It was either his younger brother (don't know how likely this is, but could have been due to him speaking out against the regime or the paranoia of his brother)
or it was another party (someone could have wanted to scare him over the missile fiasco, or spread propoganda against him but his reps pretty crap as it is)
unless he was involved with drugs or the mafia/triads which could open up another possibility.
i personally cant think of a reason his half brother would assassinate him, he had nothing to gain from it. was he leeching of north korea?
Well he was known to be 'westernised' within NK so i guess disgruntled people might look to him for hope as the successor of Un?
As for being involved in organised crime, he was under too much surveillance to get away with anything like that. He pretty much just visited red light districts and stuff and was pretty open about it. China was paying for his life of debauchery and that came at the price of his freedom and privacy as well as whatever he had on NK, which i imagine is not all that much.
I doubt anyone but NK or China was involved as assasinating him doesnt really mean anything to Kim Jong Un.
My money is on Un given he had his uncles killed when he was paranoid that the people of NK may place more faith in them. While his uncle could have been planning something, he was a significant asset to Un being a senior military general (if i recall correctly) and it was a risky move considering he had just taken power and was unproven as a leader.
That said, its pretty hard to try and put reason and logic to the delusional and paranoid.