Someone was suicided. Try a journalist, community organizer, etc. Maybe they were killed by a Putinesque foreign government, or maybe it's a situation like Mexico where journalists keep being killed by cartels in bed with the government. Or maybe it's only made to look like those things. Whatever.

You could go with your real estate guy. But he's not a real estate agent - those people aren't really powerful. He's more of a real estate speculator, someone with a lot of capital looking for returns. Like the people who buy property in Vancouver, BC or Manhattan as an investment vehicle. Like a bigwig developer with connections to city government.

Maybe he needs a place to put dirty money where it won't be taken away quickly. Like maybe the money is an account that is actually under investigation by serious kinds of police. Maybe it is payment for a load of slaves or something.

So he has this system of proxies and blinds. He's trying to buy real estate and sell it off. He's becoming increasingly nervous and desperate. So he starts playing dirty (as you say being ruthless about taking down this slum). Then this journalist takes an interest. and he panics and has the journalist strangled with too obvious a connection back to himself.

So the PCs get some clue about this. They clearly realize the journalist didn't commit suicide. The forensics make no sense. But nobody wants to deal with it (either a chilling effect or payments taken) so it's up to the players.

They don't know how far up it goes. They probably blithely follow this trail of clues that preserves them from the deepest possible shit, for a few sessions.

So the journalist leaves behind info which says that the guy is up to something fishy and wrong. The party gets this. But they don't see the whole picture. They discover the plot to mess with the slums, but they don't know about the dirty money or where it came from.

So maybe he himself is actually just a tool of bigger fish. He did something for them - not directly related. They traded him some dirty money to get rid of it. They played him. But he was too stupid to figure it out, he's just too greedy and incompetent, so he's just desperately grubbing for the best return and the slums are his desperate idea. And this makes him easy enough for the party to trace.

If the party kills him, the BBEGs are happy. If he ends up in prison, maybe they have him killed on the inside to prevent the issue from escalating to them. Killing a journalist was a bad move.