Russia did not hack the election. They ALLEGEDLY, according to the all reliable CIA, who surely do not have their own agenda, supposedly they were the ones who leaked the emails to wikileaks and possibly spread other misinformation. All this means is the Russian government is effectively better at subverting the will of the American people than the US government is. This does not mean Trump was not still legitimately chosen by the people. Though perhaps under false pretenses, that's nearly true for any election. It is no grounds for Trump to not be sworn in.
However, I don't really think it matters anyway. I don't believe there is currently a single person on this planet that's holds the real power that Trump does. That is, power over the people. And denying him the presidency would only solidify that power.
There is an immense transfer of power happening right now that goes way beyond Obama -> Trump. It's 2 sides of the same coin for sure, and I do believe it is for the best, although still not as great as it could be. That would be the coin getting tossed away forever, which neither of them want and is why this is being handled so delicately. It's MAD politics, simply.
But believe it or not, denying Trump the presidency would increase his influence 100 fold. He would have more power as the illegitimate president than as the actual sworn in, because then he gets to play his own game by his own rules. If Obama nullified the election results and declared himself or someone he appoints to be in power until (TBD), do you think Trump will not immediate take to Twitter and declare himself the Official Leader of the Free World from a Beyond the Control of Washington?
At that point, assassination would only make him a martyr.