I have to disagree with everybody - I never found Adam the least bit sympathetic. He'd attack anybody and everybody, as Julia pointed out, he tried to kill every single person he met. He killed poor old Sam Evans for little or no reason, and he always hated Barnabas because he beat him, but he conveniently forgot that the reason Barney beat him was because he was in the middle of killing Willie. When Nicolas started filling his head with bad information, Adam took to it like a duck to water. Force Carolyn to love me? Awesome! I'll get right on that! I might have had some sympathy for him if there had been the least bit of hesitation before he attacked Carolyn, but there wasn't. And it certainly didn't help that he was always whining to Barnabas about how nobody loved him. He was going to kill Vicki to take away the woman Barnabas loved, but of course she had dumped Barney and gotten engaged to Jeff Clark. Not to mention Barney's earlier problems in the romance department, involving becoming a member of the undead for two centuries. Adam whining about his lack of a love life - to Barnabas of all people - struck me as utterly pathetic.
I'm undecided about Rodan. The script would have probably needed to be changed in order to make his character sympathetic, but on the other hand, he didn't do what little he could to make Adam anything but a cold, scheming beast.