Speaking from a U.S. perspective (which may or may not generalize elsewhere), trademarks can be used by different companies when they operate in different industries. An answer on Avvo by Kurt Van Thomme captures the important question when using an existing trade name in a new industry:
The question in these kinds of circumstances is often whether the goods or services offered by the two companies are sufficiently related such that a consumer would be likely to think a company providing the first product or service would be reasonably likely to provide the second product or service also.
Kurt's answer includes an example where the name "Pioneer" is used by a seed company and an electronics company. The two products are unlikely to be provided by the same company, so there is low risk of consumer confusion.
In my layman opinion, it seems unlikely that J.K Rowling or Warner Brothers would use the trade name Voldemort to market a software development tool (as it seems unlikely they'd market a software development tool at all, unlike a complete video game, which would be a more likely product), so you could legally use the name insofar as it is not likely to cause customer confusion. Note that you still might attract negative legal attention if the trademark holder is particularly litigious, so when I say "you could legally use the name" I mean that you could probably win a lawsuit (at whatever legal fee costs) if one were filed against you.
However, some marks are regarded as "famous" or "well-known" in some jurisdictions. In that case, the mark is afforded much broader protection, and your ability to use it in a different industry is greatly diminished:
Famous marks are those that enjoy a high degree of consumer recognition in a particular jurisdiction or in a specific field of commerce or industry. However, few trademarks enjoy the status of “fame.” Examples of marks held to be famous in certain jurisdictions are COCA-COLA, KODAK, WIMBLEDON and VIAGRA.
For example, you could not start a business selling automobiles under the brand name "Coca-Cola" even though it is tremendously unlikely that the Coca-Cola Company would branch out into selling cars. The mark's status as "famous" would still allow the Cola-Cola Company to succeed in stopping you from using that mark in commerce, even in a vastly different industry. If the trademark holder of the name Voldemort succeeded in persuading a court that the mark met the jurisdiction's standard for a famous mark, then you could not use the name even for your software development tool.
Note also that because trademark is intended to reduce confusion over the source of a good or service, trademark holders can lose their trademark by failing to defend it from confusing uses. Therefore, some trademark holders aggressively pursue even borderline cases to ensure they don't endanger their trademark.