1)
What is the meaning of "va'Yshalach Razon b'Nafsham"?
1.
Rashi: "V'Af Hashem Charah va'Am" (Bamidbar 11:33).
2.
Radak: It is the death of those who lusted. It says Razon, for they intended to satiate their lusting Nefesh and fatten it - the opposite occurred.
2)
Why did He fulfill their request, and also strike them amidst anger?
1.
Malbim: Their request was not merely amidst desire. If it were, they would have refrained once they got their desire! Rather, there was also denial and heresy - they doubted His power and ability. Therefore, they did not regret, and died in Kivros ha'Ta'avah.