This is put into a better light with the newest eppy of Angel (S5ep8).
Seeing Spike gushing over how cruel Angel could be (beating the groom to death with his own arm), and having Angel giving the equivelant speech to "bored now", was absolutely chilling!
With Spike, I got the distinct impression that the railroad spike thing was simple revenge. Didn't his detractors say his 'bloody aweful poetry' got into their heads like a railroad spike, or somesuch? I can't remember the exact wording, but I seem to recall something like that. It gave me the impression that Spike had later hunted them down and brought their illustrations to life. Evil yes, but hardly on the level of the spontaeous bloodsports that Angel would engage in to sate his jaded tastes.
Darla? Well, you know Darla: Master's Pet. Vampires are ever mirroring who they were in life, but a twisted and evil version of it. Darla never had her own direction, she was ever the pawn, being used to someone else's profit. When we see her come back from death in Angel, the torment of her soul shows us a true glimpse of what she has always been: A victim. Even at her most evil, she was always someone's victim in one way or another. Angel liberated her from the Master for a time, but only to serve as her new inspiration, yet another person she could follow blindly. She's a tragic character.
With Angel, his sadistic ambition quickly outgrew any controls others wished to place on him. He was the most feared of vampires: Evil at it's purest; Evil with drive and creativity and an unquenchable thirst for the suffering of not only his victims, but that of his allies as well.