p210

I think that all three parties were involved. After realizing that Kirov had more favor in the party than himself, Stalin was looking for a way to get rid of him. The assassin obviously had some issues that involved Kirov. The assassin spent some time with a NKVD officer and was an active party member. He most likely told the officer of his troubles in passing, with the officer being aware of the plan to get rid of Kirov. The officer then took advantage of the situation to alert Stalin of using the assassin to kill Kirov, since he was thinking of doing so anyway. Then Stalin had the NKVD set up the assassination for Kirov and gave the assassin, unknowingly to him, all the opportunity he needed to do their dirty work.