The reality of the story is actually kind of sad. I don't know why he turned up the way he did. But he was actually a really great artist with such great potential. The truth is he was socially immature and mentally unstable, so he had no idea that what he shared was not the kind of stuff you want to share on the internet if you want to make a good public impression. Of course, he was much younger man in those days. He was about post high school age when it all went down. He hadn't even hit 21 yet when his world came crumbling down and was shattered.

Now, there are stories about the things that solidmario did and was capable of, and some of it may be true, but there's more to every story than it seems.

And I will tell it. And just so you know, everything you read here. It's true. All of it.

A creep he was indeed. If only I or his friend who showed him that website had known he'd be that way.

The sad things is that I believe some of these people come around with good intentions but put their talents to bad uses. Especially because of cases of pre-existing disabilites or something.
So, here's the story I have about solidmario. He had already been a simpsons fanboy since he was in the 7th grade. Bart had already been his favorite character around that time, and he enjoyed the show for what it was at that time. His childhood was a happy and innocent one as far as I know, but he was already developmentally delayed especially because of disorders such as autism, and Asperger's syndrome, but he was a smart intelligent young man in his youth.

He often loved doing simpsons pictures on his computer as a hobby among other things in his spare time when he was a teenager. And a friend of his discovered his talents with his hobbies, recognized potential within him and introduced him to deviantart. So he joined in about 2009 and went around and met some people and made at least one or two friends here and there, but finally became an active artist in mid to late 2010. People recognized his skill and he was well respected by several Simpsons fans because when he started out, his artwork was a bit more sweet and innocent at first. For the first time in his life solidmario was truly happy with himself. He'd found people who liked him for who he was. People who encouraged him to develop his talents. He was so nice and friendly to people who had befriended him at first and when he said Bart was cute at that time, it was a different meaning. Back then he meant like "puppy dog" kind of cute.
Over time, however, things changed.

 He became cold, severe, dark, foreboding and rather moody at times.

Although he had everything his heart desired, By early to mid 2011, he was different from his normal innocent and respectful self in his home life. He became spoiled selfish and unkind.

Meanwhile on DeviantArt, He became more immersed into his artwork than ever before and as he grew it became more and more disturbing and strange. He began drawing Bart in somewhat suggestive and provocative looking situations more frequently. Especially in the nude or in underpants more commonly. He already and several plain old topless Bart pics, and a few nude Barts already, but not the point of anything bad at that point. But as the months passed it got stranger and stranger. When he brought Fry into the mix it was supposed to be some kind of set up where Bart looked up to Fry like an older brother he never had or a father figure who actually showed better support for him and wouldn't abuse him like Homer would by strangling him, but sometimes it turned out to look just as creepy and weird looking as most of his other weirder artworks.

He ignored the advice of other fellow deviants who tried to convince him of what he was doing wrong and how unnatural and creepy his artwork was becoming and argued sometimes violently with those who were freaked out by his more disturbing stuff. Some of his friends tried to ask him to do other characters other than Bart for a change, but Bart was all he truly cared about and he felt he was being stifled. As time passed things just got harder and harder for him.(edited)
Eventually after Blackbustercritic  humiliated him and he was forced to close all his accounts, he finally had realized what he had done. He realized he was too "weak" for deviantart, so he vanished. His friend who introduced him to DeviantArt offered to may be get him some help to learn how to be a better artist and improve upon himself as a person, but solidmario felt it was too late for him. He fled before anything could be done to help him. And he has not been heard from since.

So much potential, all wasted. or so it would seem.

He could never do Simpsons fan art again because of the video Blackbustercritic made about him. He wanted to go back to other dreams he had for maybe being a YouTube star, which unfortunately BlackBusterCritic had also put a strain on, but he realized that if everyone was going to see him for the sick inhuman monster he behaved like, there was no point in being anything else.
His dreams were crushed and his own life was ruined by his own foolish mistakes which led to poor choices that bit him in the ass in the end.

That's why he never came back.

I think it's safe to say that basically his own addiction to DeviantArt and The Simpsons destroyed his life. But, the cause of his unnatural lustful and creepy behaviors is still an even bigger mystery. I know you were expecting more, like where he came from and what introduced him to the simpsons in the first place, but unfortunately, my memory of that is lost like him and everything he made.

Now solidmario's creations, both good and bad are all but extinct.

I think the friends he made could have been a bad influence on him.
Perhaps it's a possibility that maybe websites like rule 34 and other places like that might have affected his judgement too. But at least he never uploaded anything explicit. That's a sort of good thing.
Despite how friendly and nice a person he was, the evil temptations inside could not be held back and therefore his mind and mental well-being corrupted. Thus leading to the unfortunate consequences of his actions.

As for why he had Fry, his idea for Bart and Fry was supposed be about them bonding like brothers. It really was meant to be that way, but unfortunately it still did manage to look kind of strange. I wonder, the same fetishes that he inadvertently expressed as such for Bart he may have had them for Fry too.

But what happened to make his mind so corrupted, I'll never know. I don't think anybody ever will. I hope you don't bear a grudge against me, but I really kind of feel bad for him seeing as he had such potential.

But it was all wasted. If he had used it right, he could have been so much more.
He used to Draw Bart and Fry doing all kinds of things together. From jumping on couches to having a friendly (not gay romantic) walk on the beach to even Fry looking after Baby Bart.

I think maybe Futurama and The Simpsons weren't meant to be come together in a crossover for him at least, but at least he tried to make anything possible for his world of art. He always believed the only way to achieve the impossible is to believe it is possible. And look where that landed him up. It's just sad.

I think all he ever wanted was to make a difference in the world with his artwork and make something he could be proud of. Alas, 'twas not meant to be.
It was a horrible time for me and him both. When they were so cruel to him, and I could hardly bear it.