Like many here, I agree there was sometimes some very sloppy writing in the series. However, I also believe some folks are complaining about things being "out of character" when in fact they are very much the opposite.
Barnabas Collins was a complex young man in 1795. All that gentleness and devotion was the surface, but even as a human being he had a awesome temper. Let us also not forget he had an affair with his fiancee's personal maid! While he clearly loved, even adored Josette the simple fact was Angelique always got under his skin, always seemed to elicit a powerful emotional--and physical--reaction in him.
When he became a vampire, Barnabas underwent a change. He often was vicious. His first act was to taunt Angelique, making sure she was afraid before he strangled her. Many recent vampire tales--from Bram Stoker's Dracula to Buffy--pretty much make the case that vampires are a case of demonic possession. Methinks you could make that case with Dark Shadows as well. Or in Freudian terms, a vampire's Id becomes very very powerful at the expense of their ego. Barnabas did terrible things as a vampire, many of them at least in theory unnecessary. Yet he wasn't without a conscience, so he also got the trauma of personal guilt on top of everything else that happened (which all came down, btw, to his having cheated on a woman--or two women, depending on how you look at it).
I interpret Barnabas as having eventually entered into a kind of trance while chained in the coffin, spending very many decades simply reaching out with his semi-conscious mind to summon someone to release him. Hence the strange "feeling" around the mausoleum. When he did emerge, quite frankly he was a nutter. He in every way he could manage refused to live in the 20th century--refusing to have electricity installed in the Old House, trying to turn a waitress into Josette, etc.
These are not rational things to do.
But when he was cured, Barnabas also began to regain his sanity, along with a couple of boatloads of guilt. His rather hystrionic personality did not help with that--in simple terms, Barnabas is and always was something of a drama queen. If there was a dramatic gesture to be made, even if it made little sense, he was at the very least tempted to do it!
Mind you, he also became a much darker person every single time (twice, wasn't it?) he became a vampire. Or (more often) he travelled back in time into his vampiric body--when he was demonstrably more violent and reckless.
Specifically, I think his emotional instability and the inherent violence of his undead nature do much to explain the murder of Carl.
Vis-a-vis Angelique, I've always felt that Barnabas to some extent--and with some justification--blamed himself for her actions. More, he always had strong feelings for her, as she did for him (although both tried to pull away at one time or another). Given that they both changed, both matured, both had a great deal of guilt with which to live and each ended up sacrificing so much--I believed Barnabas could indeed have fallen back in love with Angelique. While I don't think that makes her the "only woman" he ever loved, I totally and absolutely believe Barnabas would say something like that (see Drama Queen comment above).
