I have read a WIDE range of varying interpretations of Sesshoumaru's character. I have a list of well over 100 fan fics with Kagome/Sesshoumaru that are MA rated alone, and others not MA.
I have seen him portrayed from the beginning as a nice, wishy-washy guy, and like PRVN says, that just doesn't seem Sesshy.
I do prefer to see a romance/relationship being eased into slowly. There are some stories that ease far too slowly, while others rush into things. Theirs is a relationship that can be difficult to writers because of the fact it is so outside the norm, the two have nearly complete conflicting personalities, races, and backgrounds. There is a very fine line between making the story believable, and make the story move too slow.
Though I am not a fan of it, there have been some VERY well written stories (at least in my opinion) where Sesshoumaru has abusive characteristics. Though I do favor the happy ending stories, there are times I do enjoy the realism, the plausibility, and the accuracy in relation to the times.
For example, Texanlady's
The Breeder is actually a VERY realistic idea for that time in nearly any culture. Men were often quite brutal to women. If you ever get the chance, watch the movie,
The Dutchess. The Relationship between the Duke and his wife are actually very accurate in most marriages around the world, sometimes even to the current day in some societies.
Another good example is
Youkai Society by HeartlessEnchantress. Most marriages, in this case mating, nearly always had some sort of benefit, whether it be dowries, political aspects, or linking royal families, all classes from rich to poor (though lowerclass people usually got a fairer pick, and those of nobility were usually forced into marriage for some sort of gain for their family) had marriages made only for gain.
I am one who actually loves history. The author, Bertrice Small, writes historial fiction/romance novels. She is a wonderful author, and her detail and accuracy are astounding. But again, you do see a lot of the darker aspects of humanity. And while all of her characters eventually reach their happy ending in some form or another(with the exeption of one book, but that one is complicated), she does so realistically and accurately. Most of her books take place during King Henry VIII, and there is much death, betrayel, losses, and more.
So yes, I do believe Sesshoumaru can have a darker side. It is all in the interpretation of the writer/reader. I have seen very well written fics that depict him as either or, and both have been VERY believable.
I personally can see him in either direction. He could potentially be an abuser if it meant he maintained his dominancy. I DEFINATELY can see him being a strict disciplinarian with any "children/pups." Again, historically speaking, corporal punishments were most common for that time. I can see him as personally adminstering such punishments that would in modern times be called abuse.
And yet, I could also see him as repulsed to the idea of violence outside of the battlefield, and especially in regards to women and children. Though he nearly kills Kohaku for his slights to Rin, you know he is doing to to protect what he may very well consider his family. Even in modern days, I know of fathers who would go to such extremes to protect their own families if necessary. Even the few times he tries to attack Kagome can be explained. When she pulled out Tetsusaiga, he may have felt she was a threat to his rule as Lord of the West, and especially when she gave it to Inuyasha, it may have proved her as a threat. He may have felt that he needed the blade to protect his lands. The same applies for when he attacks her for attacking him. Not only is it defense, but it is a threat to him, the Lord of the West, and thus the protection of his lands. Other than those few times, Sesshoumaru, while aloof as PRVN says, and detached from the rest of the world, only seems to harm those who either get in his way or threaten him or his family/pack.
Again, I feel it all depends on the writer. I feel, if the writer can make it believable, even if it is or is not a happy ending, then kudos for them. It is difficult enough to portray characters in an original fiction, but to take characters from someone else's work to create another idea and develop those characters as accurately as possible is no easy feat. It is commendable to give any believable and moderately accurate personality to these characters.
And also, in my opinion, it is good to keep your mind broad and your options open. "Don't knock it til you try it." Like I already mentioned, I am not partial to most darkfics. I was not even sure I was going to like
The Breeder. But as I read it, I actually found I enjoyed it. Did I appreciate the abuses? No. But did I enjoy the story itself? Very much. Sometimes you may miss out on some wonderful stories because you felt you wouldn't like it. Try a few chapters, and if you don't like it, well, at least you tried.
I tend to favor more adult rated fictions because I like the more realistic aspects. Very rarely in life will you find entirely G or even PG-13 rated aspects that are accurate and in truth reality. Especially having lived through a lot of reality, I find I crave a more realistic approach. Does that mean I neglect the "fairy's and butterfly's" aspects on life? No. But I find that do not appeal to me as much. That does not mean I don't read them. I actually have read (and maintain in my gigantic list of fics I have read/would like to read) that are not more adult centered and are actually accurate in reality as well as well written.
Granted, this is all my opinion, but I do try to approach this in a very logical and methodical way. Whether you agree or not remains to be seen, but one cannot argue the logic and the potential ideas.
I will admit to being curious to responses to my 'essay' though!