Kind of answered this one yourself. There’s no clear motive other than “weird loner gambling addict decides to commit an atrocity”, and there’s just not much to say on the matter.
i would advocate digging through his entire posthumous private life, regardless of how apparently relevant any of it is, and extensively interviewing his friends family and colleagues. anyone's life can be examined to the extent that it fills a book
Advocate to… to who? Who do you want to do that. Me? You? OP? The police?
They have already compounded a staggering amount of information onto this man who, by all accounts - was simply one of the most boring and bleak men to live. No one has simply “forgotten” or “not thought” to do that. There’s just nothing there.
And what do you mean “posthumous private life”?? The dead do not have private lives. You can interview his ex girlfriends and siblings as much as you want, none of them will suddenly have more info on him.