The guy was already retired for over 7 months so assassinating him was pretty pointless, but Xi may have done it anyway to consolidate power. If Xi was worried about him restraining power why not do it during the 10 years where he actually had influence, although evidence shows that Xi may have been forcing him out of public spotlight during that timespan. Yeah, there's conspiracies about his death floating about in the country but that just means the assassination hypothesis is even more absurd, even though it is possible. Why would anyone risk a public backlash for essentially no gain, unless they were despotic evil dictator?