If you find the players playing evil PCs to be abhorrent, of course you shouldn't let them play evil PCs, you need to end this campaign immediately and start another one with a requirement that all the PCs are good guys.
Personally I prefer the 'realistic consequences' approach - ie, the Wages of Sin are Death, for almost all evil-psychopath types in a 'typical' setting like the one you describe. You can also insist on describing the NPC victim's reactions (the fear in their eyes, their screams, mothers clutching frightened children, frothy bright red blood from the mouths of the dying etc etc) in squirm-inducing detail, most evil-PC players don't like to really consider what their PCs are doing to 'real' human beings; those few who would enjoy it probably aren't the kind you would want to play with.
So, either:
1. End campaign now
or
2. Continue, but show PCs the consequences of their actions - being hunted down like dogs by the _real_ heroes, and (if you wish) show them just what they're doing, whether they want to face it or not.
Personally I prefer the 'realistic consequences' approach - ie, the Wages of Sin are Death, for almost all evil-psychopath types in a 'typical' setting like the one you describe. You can also insist on describing the NPC victim's reactions (the fear in their eyes, their screams, mothers clutching frightened children, frothy bright red blood from the mouths of the dying etc etc) in squirm-inducing detail, most evil-PC players don't like to really consider what their PCs are doing to 'real' human beings; those few who would enjoy it probably aren't the kind you would want to play with.
So, either:
1. End campaign now
or
2. Continue, but show PCs the consequences of their actions - being hunted down like dogs by the _real_ heroes, and (if you wish) show them just what they're doing, whether they want to face it or not.