Ttyjukl

In the film Harry Potter and the Prisoner of Azkaban, Professor Lupin severely reprimands Harry Potter after saving him from Severus Snape.

-- I haven't the faintest idea how this map came to be in your possession but quite frankly, I am astounded that you didn't hand it in. Did it never occur to you that this, in the hands of Sirius Black, is a map to you?

-- No, sir.

-- Your father never set much store
by the rules either. But he and your mother gave their lives to save yours. Gambling their sacrifice by wandering the castle unprotected with a killer on the loose seems to me a poor way to repay them! Now, I will not cover up for you again.

It seems natural that Lupin was angry with Harry, as there is a murderer at large and Harry walks at night without any protection. But why did he think that the Marauder's Map makes the matter worse?

For me, the chance that Sirius Black will ever lay his hands on the map is very small, while the protection that the map gives Harry by allowing him to see who is around him is considerable.

The equivalent scene in the book is as follows, and sheds a bit more light on Lupins thinking.

"I don't
want to know how it fell into your possession. I am, however,
astounded that you didn't hand it in. Particularly after what happened
the last time a student left information about the castle lying
around. And I can't let you have it back, Harry."

Harry had expected that, and was too keen for explanations to protest.

"Why did Snape think I'd got it from the manufacturers?"

"Because...," Lupin hesitated, "because these mapmakers would have
wanted to lure you out of school. They'd think it extremely
entertaining."

Lupins primary concern is not Sirius getting his hands on the map (Sirius has no real need of it, he knows all of Hogwarts and can stalk Harry in dog form, and Lupin should know this), but that the map will draw Harry out of Hogwarts, where Harry would be unprotected. Worse, Sirius knows all these routes and is likely using them.

While Lupin isn't (or shouldn't be) concerned about Sirius getting the map, he's also not happy that Harry didn't consider it a possibility - as @Jenayah points out in the comment, Sirius had previously gained access to the Gryffindor dorm because Neville left a list of the passwords out. Harry really should have learned from this, but kept freely using the map regardless, despite knowing that it listed several hidden passages in/out of the castle.

Interestingly, the film seems to be combining two different sets of dialogue here (Lupin and Snape, of all unlikely pairs!). The scene continues as follows:

"Do you know them?" said Harry, impressed.

"We've met," he said shortly. He was looking at Harry more seriously
than ever before.

"Don't expect me to cover up for you again, Harry. I cannot make you
take Sirius Black seriously. But I would have thought that what you
have heard when the dementors draw near you would have had more of an
effect on you. Your parents gave their lives to keep you alive, Harry.
A poor way to repay them, gambling their sacrifice for a bag of magic
tricks."

And earlier in the book Snape tells Harry

"Your father didn't set much store by rules either," Snape went on,
pressing his advantage, his thin face full of malice.

Totally reasonable and in-character, Snape's dialogue coming from Lupin. Absolutely fine.

Well. As I see it, the answer is in the exact piece of dialogue you quoted.

Did it never occur to you that this, in the hands of Sirius Black, is a map to you?

Lupin is worried that Sirius gets hold of the map. Which is far more likely when Harry uses it in or even outside of Hogwarts than if it's locked away in a teacher's safe or something.

Since Sirius is one of the people who created the map, he would recognize its value in finding Harry and know how to use it, in contrast to Snape.

If Sirius got the map, he would be able to know where Harry is 24/7.

So even though he might know the passageways already, he would be able to always know where Harry is, finding when he is alone and unprotected.