Fischer was afraid of Karpov?

@63
Around 1982 both Karpov and Kasparov would have been stronger than Fischer.
The approximate strength of Fischer is above that of Spassky, who was still active.
http://www.chessmetrics.com/cm/CM2/PlayerProfile.asp?Params=199510SSSSS3S062745000000111000000000000010100 

The fear was in him. He said that if he played Karpov, he was going to insist on a long match. After not playing for three years, he was very concerned about how good he would be.

it was like no games played and Karpov become world champion that was easy

In 1975, my money would have been on Fischer. He was the better player and his form would have returned. Fischer's rating in 1992 was 2660 - only 125 below his peak 20 years earlier - so he would have surely been the strongest player in the world in 1975, and would have been competitive with the 2 K's even in 1982.

To explain the last point, it is likely the amount of strength lost in 1972 to 1982 would be less than half that from 1972 to 1992 - typically the fall in rating accelerates.

It is possible fear was a part of it.

However as he had already done what he really wanted...beat the Russians. , he may have felt there was nowhere else to go.

Any further matches v Russians would therefore, in his somewhat deranged mind, be pointless.