FLYGVA is the real expert when it comes to Düsseldorf, he is correct with everything he has said.

Let me assure you that Düsseldorf is pretty safe.

Regarding night photography, there are plenty of spots outside of the Christmas markets you want to look at. The Rhine river and especially the media harbour (which also holds a Hyatt and Radisson Blu) is definitely worth a look for you.

Don't be distracted by small "christmas markets" that are all over town. (basically a few stalls here and there) The "real" ones are downtown like mentioned nearby city hall (Rathaus).

To be honest, you might think about going to Cologne since the city holds way more large Christmas markets but it would involce a train ride of approx. 35 minutes each way.