http://www.bbc.co.uk/news/business-35998801
Switzerland
Hong Kong
USA
Singapore
The Cayman Islands
Luxembourg
Lebanon
Germany
Bahrain
United Arab Emirates
Panama seems to be down at the low-rent end of the financial secrecy market.
So no - the OP is not only not plausible, but raises more questions than it answers.
Practically, I'd expect a Russian hack to specifically target the tax affairs of Putin's enemies.
This may yet happen now, with Russian and/or Chinese hackers targeting the tax secrets of prominent Western politicians - which would be an interesting development for everyone.
Personally I am a libertarian. I am concerned by the death of privacy and the increasing overreach of governments. I don't really see this drive toward transparency and bullying of countries offering financial secrecy as a good thing.
I have seen or heard of many people who have gone through WW2 and have hidden money in Switzerland. Not to hide money from the taxman. But because they have seen how the governments can easily turn against their citizens and decided they needed a plan B. This concept of "nowhere to hide" is a bit concerning.
Presumably you are talking about Roberto Calvi and Alexander Litvinenko.
While two murders that could be linked to the Russian state are two too many, describing this as 'regularly' seems a bit of a stretch.
That being said, I agree that Putin is near the top of my "people who should not be messed with" list.
Putin was at the top of the revelations, even if the news was unsurprising (but like the Snowden documents, it's not because we all suspected it that the proof is any less valuable).
If the guy who did it is not US authorities, I would not not sleep well if I was him. The police in London regularly picks up the hung or poisoned bodies of those who dared to attack or expose Putin...