Nowadays, it is quite popular to download music, films, series, etc without paying and for free.

Add "for free" is a bit redundant, so I think the sentence might sound better without it.

Some people think that it is a some kind of a crime,; other ones are sure that it is not.

You technically shouldn't connect two independent clauses with a comma because it's a punctuation error called "comma splicing." You can use a semicolon (;) instead, like how I corrected the sentence to. There's other ways of fixing it too, which you can find if you do a quick Google search of "comma splice."

I want to explain why we can say that this is a crime as well as stealing.

Your original sentence is good, but if you want it to be more concise, you could try "I want to explain why this should be called stealing."

Firstly, if something costs a money, that means you have to pay to get it.

For instance, if you want to buy some products from a shop, and of course you have to pay the money.

Secondly, it shows disrespect to the people who worked on these projects, regardless of whether we're talking about the music or the film industry.

I think it sounds better as "the people," since you're pointing out a specific group of people. Then, I added "whether we're talking about..." to clarify what you might have meant by "regardless of."

In addition, illegal downloads affect artists, actors, musicians and others who madeput in a lot of effort.

To sum up, I believe paying for the music you are listening to, or paying for the films and series you are watching has to be a common thing, and that it is really significant and important to a team who worked hard to make it all.