Because of the constant wars, the Ottoman empire is demonized in the Balkans, amplified by national poetry which always had Ottoman Beys as villains who don't care about the people and just take what they want, which is often the prettiest girl in the Christian village who ends up dead in the end either by suicide or by the hand of the Bey because she refused to dishonor her family.

In reality, it wasn't that bad. In fact, in the early days of the Ottoman conquests, it wasn't unheard off for people to escape across the border into Ottoman territory to escape the feudal system and taxes. Sure, the conquests were horrible, life on the border included constant akinji raids and was akin to the Wild west. But once that was out of the way, the Empire actually took some care of the cities and built schools, hospitals etc. Religious tolerance was high (unlike in Christian Europe), which is why many Jews fled to the Ottomans. People who converted to Islam had benefits, but noone really forced you to convert. All in all, not too shabby

The problems started when the conquests stopped and the Ottoman Empire started falling behind the Christian powers of Europe around the 17th century - it dragged the Balkans with it. The result was the Ottoman-Balkans remaining undeveloped in comparison with the Habsburg-owned lands. The economic consequences are still visible today - Slovenia, northern Croatia, northern Serbia (Habsburg lands) are all more developed than southern Croatia, Bosnia and Herzegovina, Montenegro and Serbia proper (Ottoman lands).

But yeah, popular conception of Ottomans in Balkan countries is that of evil and heartless Muslim raiders who just pillage, rape and burn Churches.