Why do heterozygous individuals have increased resistance to malaria?
Very interesting explanation has been proposed by Ferreira A. et al. 2011. Briefly, red blood cells of individuals carrying homozygous point mutation in haemoglobin beta-chain tend to be more prone to disruption.

Red blood cells are full of haemoglobin, so once their membrane is damaged, they release free haemoglobin and heme to the plasma. This is called intravascular haemolysis. Too high level of free heme is toxic and you can read about its consequences, for example here. 

In the plasma of individuals with heterozygous mutation also there is an increased concentration of free heme, but definitely below harmful level. 

However, it is enough to trigger mechanism that protects us from toxic effects of free heme. Heme oxygenase-1 (HO-1)catalyses heme to biliverdin, iron and carbon monoxide (CO). CO makes the vascular environment very unfavourable for Plasmodium.sp and probably protects from infection. 

The study was performed in the mouse model.