Daoud was initially fairly socialist at heart, which is why he was able to gain power because he was supported a bit by the USSR. When he distanced his country's need for the USSR, they feared it would result in his reliance on the West, which they understandably had a problem with. When he was overthrown, he was not replaced by Amin, he was replaced by Taraki. Taraki was later overthrown in a shootout by Amin. Amin also wouldn't take soviet advice on government structure and roll in economy and whatnot, which is why they came into play much more heavily. No post monarchic government in afghanistan has been able to support itself and develop a strongish working economy, which the soviets were trying to help it do, and which is why it relied so much on it's neighbor. While the land reforms weren't popular, they weren't well thought out or implemented and could have used soviet support, which may not have been so anti-islam. As far as number of people the PDPA had killed or tortured, the Taliban, since receiving initial US support, has done much worse.