What didn't he do is the better question.
Let's see: threatened an artist into trying to create a banner and mascot for the website that was an in-joke in a Discord server, despite me and several other people telling him it was a terrible idea seems like a good place to start. Without the artist's consent, he announced to the sub that this mascot was coming and going to plastered on the banner, no exceptions (think the current subreddit, but with a redheaded caricature that has no connections to Grumps). To the point that he ended up pissing off the entire subreddit. Afterwards, the artist in question as well as several of RG's frequenters left as they no longer felt comfortable being there or being associated with the people there.
(Bear in mind: the server itself ended up being a complete disaster, with one dude always coming in drunk in late hours of the night insulting several people over the course of several months, changing his username over and over and discussing Nazi/alt-right propaganda.)
Immediately after, the artist wanted them to stop using emotes/the mascot as they still planned to use it without his consent or permission and had to plead on said server to get them to take it back. Valkuma, pissed off about this, then immediately blamed anyone who supported them and made a lot of people leave out of, if nothing else, sheer indifference. This included, and especially so was pertained to,
myself, despite me literally doing nothing. Like, actually nothing to the point I just left.
They also tried to meme Mr. Burns (yes, The Simpsons Mr. Burns) for some reason in an attempt to stop any discussion from happening, and at that point, we just stopped because it was a lost cause. Still using the artist's shit, don't know what they plan on doing with it, but none of us care anymore (even though Valkuma clearly did: I wasn't even reminded of this still going on until literally a week ago, I had just forgotten about it after all this time).
All for him to immediately kiss ass the second anyone's videos started gaining traction on the sub, to the point he wanted for a collaboration with people on the subreddit (EDIT from the future: one of those including LS Mark pre-video phase IIRC). Nowadays, in those circles (they didn't even check to make sure the people they were insulting were still in the server) they apparently regard that group and that anything done thereafter as being done out of spiteful justice or whatever the fuck. It doesn't matter.
Aside from that, he's the entire reason that subreddit is as cancerous as it is: heavy, heavy alt-right leanings to oppose the non-edginess of the Main Subreddit, makes subreddits in order to gain infamy and nothing else (to the point he tried and failed to launch a hate subreddit for SuperMega), acts like a complete and total jackass to anyone with a differing opinion to him, constantly allows and encourages others to do actions that cause them to be banned from channels with Grump-related content and the Main Subreddit, implemented a system that will passive-aggressively flair you in order to embarrass and humiliate people he deems stupid, and may or may not be way too open about his overt fetishes, sexual or otherwise.
Oh right: he also vehemently refuses to cooperate with fuckin' anyone or use any posts that could better help the subreddit grow (one time, he literally just shilled the Discord again for no reason other than a passing mention by the Grumps of a Grumps hate Discord), or better its standing/image within the Grumps community. RantGrumps, for as exceptionally enraged as they are, has to do the leg work
for him when his other couple of moderators have better things to do than to be sad-30-year-old-men running a hate subreddit after the last attempt at a venting subreddit (VentGrumps) ended in the Etsy scandal being created, and the previous owner shutting the entire thing down in...anger? Malcontent? No longer giving a shit?
In short, he's spiteful edgy autism incarnate. I mean, there's more, but I'd have to dig through a ton of screens and conversations, and this thread's been derailed enough.
EDIT: Hey: you asked for stupidity, you're getting stupidity.
EDIT 2 (2021): Went back and corrected some information here for posterity, as well as removing interpersonal drama unrelated to the thread and outright completely wrong information.