While yeah, I believe it's not cool to insult the writer of these things, there IS something that I'm fairly sure anybody here can blame here without fear.
Their developmental stage, as writers, and as people.
There are plenty of suefics that are downright misanthropic, and nearly ALL suefics do not consider the happiness/well-being of canon characters with all that they barge into the canon and usurp the story focus from people who were actually trying to live their lives. So many of them exist only so a real-life person can pretend to be 'better' than they are. And so many have ulterior motives, as perpetrated by the author.
Example. There is this fic that I am desperately trying to spork at the moment. It is Legend of Zelda. What this author does is pair off ALL female characters with OCs or kill them off, and is is very obvious she is doing so so that Link's only choice of mate is the Mary Sue... by way of availability. She cuts off all possible choices. Where one would normally say 'you can't know what the author was thinking when they wrote this,' ... no. You really can. Not only is everybody immaculately paired off, when Zelda becomes available again (her OC 'husband' is killed off for the sake of making a bad guy look more evil) she is immediately and very defensively stated to be pregnant and thus can't date Link. And then when the author forgets about the pregnancy, she decides to make Link and Zelda brother/sister (half-elf-half-hylian brother sister to boot) such that they 'can't' get it on. Previously unattached characters such as Midna immediately grow husbands, not after the plot of the story, but as if they had been married all along. Midna is inexplicably the 'Twilight Queen' with a husband when there had been no hint of it in the game Twilight Princess.
The author's ulterior, selfish motives are clear. But would it be wise to say the author's a selfish person? Yes... temporarily. Not permanently. This person's developmental stage is stuck in the egotistical, selfish stage normally associated with early adolescence-- you know the one I'm talking about. The one that explains why everybody suddenly became idiots in middle school/early high school.
When I want to criticize this mindset, I usually take the Mary Sue as a manifestation of it. The Mary Sue wants to hook up with Link, so the Mary Sue makes sure all other women are hitched as to not even give him a choice.
If this author is not really that bright depends how long they are stuck in those developmental doldrums. I hate to insult authors, but if somebody is still acting like a selfish fanbrat at age 25, not all might be well with them. I have an older cousin who acts as if he thinks he is dark, paranoid, gritty and 'extreme' because he thinks it's cool... not realizing he comes across as a moron. Not due to any mental issue or brain development. Just because that's what he thinks is cool. It's really not.
Why wouldn't this 26-year-old doofus make a Gary Stu that is a manifestation of his 'screw actually being a human being, I'm more awesome than you and I am going to prove it!' attitude?
So yeah. Most suethors will grow up to be nice, quirky people. Heck, I am still in the process of growing up to be a nice, quirky person. But many of the authors writing that fanfic that makes you scream 'how could you be such an insensitive moron!' or 'you really enjoy being a horrible, manipulative person!', at THAT darkest stage of suethordom... are temporarily not that great of people.
There's a reason why early adolescence and bad suefic often go together. More often than not, it's that lack of empathy/thought that characterizes that developmental stage taking a manifestation in writing. Not all early teens are like this, but the ones that are sure do gather in FF.Net like flies on your ice cream.
***Just a note, throughout all of this, it is still not cool to insult writers, no matter what one may think of them. But I believe knowing the author's motives and mindset allows one to fully understand, to grok the story itself and allow for a funnier interpretation. It becomes a tale that wildly makes up ridiculous things in order to try and get some kind of goal rather than a story that makes one wonder 'WHY WOULD ANYBODY DO THIS?'