One person owning x% of the Masternodes is a very different thing from one person owning x% of the hashpower. Probability of attack success does not scale linearly with attacker numbers when it comes to Masternodes.

Masternodes are selected each block/transaction effectively randomly, in an n-of-m fashion.

Do the math on the chances of 10% of Masternodes belonging to one person being compromised enabling anyone to trace a transaction through for example 8 rounds of Darksend:

0.1^8 = 0.00000001.

Good luck with that.

At 51% compromised Masternodes it rises to a mighty 4.58% chance of success, vs a 51% hashpower attack which has a 100% chance of success.

If you're not getting that Masternodes are fucking awesome, I suggest laying off whatever shit you're smoking and injecting some steroids directly into your brain. Might take you a few stabs to locate it...