On one hand, the Bulgarians "signed on," with Nazi Germany when the latter was looking for Balkan allies against Russia. So did Hungary and Romania.
Hungary had been part of the Austro-Hungarian Empire during World War I, and also had an alliance with Italy (against Yugoslavia). Romania had been pro-Allied during World War I, but sided with Germany for roughly the same reasons as Finland; the Soviet Union had annexed parts of Romania (Bessarabia and Northern Bukovina) in 1940. Bulgaria had aided the Central Powers against the Serbs during World War I, and did somewhat the same for the Axis against Yugoslavia in 1941 when that country (originally an Axis signatory) "backed out."
But unlike the other two Balkan states, Bulgaria did not willingly send troops to Russia to assist Germany. Also, Bulgarians resisted Nazi efforts to round up the country's Jews, unlike the others. Finally, the Bulgarians (http://en.wikipedia.org/wiki/Bulgarians) were (south) Slavs, whom Hitler, at least, disliked.
In this regard, they might have been considered similar to the treacherous (Yugo)slavs. Or were they seen as a Turkic people that some scholars consider them to be? (Turkey had also been a German ally in World War I.)
Did the Nazis have similar views of the Bulgarians as the other (non-Slavic) Axis signatories after 1941? Or were they considered a "necessary evil" to be dealt with after the war more harshly than the others?