It's actually very easy, because only one person is compiling 'official' bitcoin client which is used by thousands users, force this person to corrupt, backdoor 'official' client and you can destroy bitcoin in just few second... and don't give me argument you 'can Compile it yourself' 90% use standard bitcoin-qt client.
Thanks to open source, backdoors is usually found and fixed, and the one how did it is recognized.
What worries me more than bitcoind is the java binaries that some wallets depends on. Those java binaries can contain anything and nobody would know about it.
It is also possible to create laws that require you to add a backdoor and another law that prevents you from being honest about it.
The best tool we have is to require all crypto related software (and even hardware) to be open source