by late 1968 Barnabas had become sort of a snooze. a vampire who wasn't a vampire...
so returning him to the "vampire state" in 1897 (a period in which Adam was not extant) was a way to give the character some "blood" again so to speak. there was a reason he always became a vampire again when the storyline shifted to another time period. you can't have a "vampire show" without a vampire.
[spoiler]so to me it's not that surprising that he killed Carl. he was, after all, a vampire again. and that is not without an intrinsic element of dangerousness and unpredictability. besides Carl was a "time travel" character. they're basically created with a built in shelf life. by the end of the period most of them usually end up dead anyways. and he was only in a few episodes and not very important to the overall plot so who really cares???[/spoiler]
all of the fuss that particular incident creates within the fanbase has always mystified me.
That's interesting because I've never seen Dark Shadows as a "vampire" show. After watching 1225 episodes, I'm still unsure exactly how to define it, but in broad terms for me it's a show about the strength of family and the power of love in overcoming overwhelming and almost impossible odds. This is quite trite and cliched, but at the moment it's the best way I can put it.
As for Barnabas, his vampirism is only a small part of his appeal for me. Angelique's curse exposes all of Barnabas' flaws: his poor and decidedly indecisive decision making, his immature and misguided love for Josette, and his dishonesty. The 1795 illustrates all this brilliantly. But I think Barnabas becomes even more interesting when he's cured of his vampirism. He's been cured, but he still has all his same hang ups. We see him kinda sorta trying to work through them throughout Adam/the Dream Curse, as he begins to start thinking less selfishly. The moment he spares Jeff Clark's life for Vicki (I think that's what happened; it's been a while) is an example of this. Fighting Quentin's ghost is really the first time we see this put into action when he works in a positive, proactive, selfless manner to protect the Collins family. As for 1897, yes, he's a vampire again, but I've always gotten the feeling that for whatever reason (increased maturity, maybe) he had a certain degree of mastery over it. In saying that (I hate that phrase, but there it is), the killing of Carl never really bothered me (from Barnabas' character's standpoint that is; from a storyline perspective it really bummed me out; I loved Carl). I've always seen Barnabas at that point of the story as sort of like a dormant volcano that hasn't erupted in a while, but always has the potential to. Those two aspects really added dimension to the character. We see the same thing in the Leviathans, which is why I like it.