This is not true. A judge could be disparaging to the defendant in court, for example - this is not corrupt, it is just highly unethical.
Corruption is "when an elected representative makes decisions that are influenced by vested interest rather than their own personal or party ideological beliefs" [1]. The key phrase is vested interests - that's what separates vanilla unethical behaviour from corruption.
OK, but why would a judge do such a thing out of the blue, with absolutely no motivation? You're assuming that an otherwise competent and objective judge would wake up one day, walk into court, and just randomly decide to do something highly unethical (we're not talking about accidentally making more photocopies than he needed or something, we're talking about something highly unethical) for absolutely no reason at all.
Now, perhaps he has a mental illness and he has begun doing things for no apparent reason, beyond his conscious control. That might be the case. I'm sorry for missing that narrow exception.
However, in practice, rational people don't just do highly unethical things for no reason. In fact, that really speaks to the very definition of a "rational" person in many ways. No, they do them because they get some sort of satisfaction out of doing them, and that is the definition of a "vested interest".
Highly unethical behavior without some motivating vested interest just doesn't exist. The vested interest needn't be financial of course, but it is still there.
My point really has nothing to do with the ruling being correct or incorrect, though. My point is that highly unethical behavior is almost always corrupt by the definition of corruption you gave because there is almost never a reason other than "vested interest" for a public servant to engage in highly unethical behavior. Even if the vested interest is something relatively innocuous like convenience, it is still a vested, personal, interest.
It is also worth pointing out that corrupt behavior isn't always illegal under that same definition (at least the portion you quoted, which seems to be a pretty "common sense" way of looking at the issue).
Additionally, even where the behavior is illegal, it is easy to imagine situations in which there would be no way to prove to a court (given that, in the US at least, the standard of proof is quite high in criminal trials) that the corrupt behavior violated the law.
This last point is one reason we have elections. Even when the courts can't establish that a leader is corrupt (quite a tricky thing to do), the public can convict the individual at the ballot box. Of course this doesn't always work, and sometimes it works backward (and it doesn't work at all for appointed judges, though in the US they can be impeached and removed by the legislature).
No system is perfect, but based on the article I am comfortable concluding, at least provisionally, that this particular judge is corrupt. Of course my opinion means exactly nothing to the Dutch people and government, but since when has an issue like that stopped us from having a rousing discussion on HN? :-)
Yes, doing something "highly unethical" IS corruption. If you are a public official and you do one or more "highly unethical" things, you are corrupt.