For starters propaganda is any information, ideas, or rumors deliberately spread widely to help or harm a person, group, movement, institution, nation.

If I put up a poster on proclaiming that "/u/Ultrashitpost is the best" it is just as much propaganda as a poster that claims "/u/Ultrashitpost is the worst."

How a memorial is presented has a lot to do with how propaganadaliscious it will be.

I'll give you two examples.

a) Monument to remember victims of specific event. E.g. victims of the Halifax Explosion, or the rescue workers of 9/11.

b) Monument to remember victims of an idea. E.g. victims of natural selection, or victims of creationism.

Do you see the distinction here?

Okay, creationism and natural selection might be a bit silly. Let's try another example:

B) A monument to the victims of Nationalism.

In A) we remember an event--which had bad consequences. In B) we remember an idea that was... bad?

A memorial to the victims of the Holocaust is about an event.

A memorial to the victims of Nationalism (a contributing factor to the circumstances that made the Holocaust possible) would do what?

An idea isn't inherently bad.

Elements of Nationalism can be used for good. Aspects of nationalism helped the United Stated rally efforts to get to the Moon. At the same time, Nationalism has been often used as an excuse to go to war resulting in many deaths. (E.g. the Franco-Prussian War, or the Invasion of Iraq and The War on Terror.)

A monument to victims of the Holocaust can help to unpack and explain the causes of a terrible event. This can be done showing facts, and avoiding bias. (Nationalism was used to fuel hatred. Bad things happened.)

A monument to victims of an idea cannot educate in the same way.