Reading HP fan-fiction is a fairly hazardous pursuit for me since my favourite types of stories ('Canon Rewrite' and 'Peggy Sue') are also the most likely to include gratuitous bashing on my favourite character. Even the 'exclude' feature on AO3 only goes so far.

Here's an example. I was reading a really nice 'what if' story, wherein Lily ended up with Severus. It was going quite well and I felt safe since the summary had specifically stated that there would be no character bashing. Lo and behold, however, my enjoyment was brought to an abrupt end when the author decided to dedicate an entire chapter to an intervention, or some such, where all of the adult main characters berated Dumbledore. Two things seemed particularly egregious; Snape referred to Dumbledore as "worse than Tom Riddle" (yes the self-same mass murdering lunatic) and Elphias Doge was revealed to have been a false friend (leaving the old man friendless and alone).

A few months later, I encountered a story in which Fawkes himself abandoned Dumbledore, and there appears to be no shortage of these sorts of stories. I don't go out of my way to read them and I don't begrudge people for writing them (apart from when they pretend they aren't) since fan-fiction often serves a cathartic purpose which character bashing is an efficient, if crude, vessel for.

So, for some cathartic release of my own (because, goodness knows, I can't get actually create something to channel my emotions into), I thought I'd make a catalogue of things these stories have in common that make Dumbledore look worse than even the least charitable interpretation of actual text could:

1. The Will. These stories invariably include reference to a will specifying Harry's placement with someone other than the Dursleys. That's fair enough, of course, but the most likely candidates are indisposed so it typically includes an almost comically long list of people with increasingly tenuous connections to the Potters or, more bizarrely, explicit instructions not to place him with the Dursleys. Dumbledore ignores the will, of course, because he's evil.
2. The Magical Core. Another one, though less common, is the idea that every witch/wizard has an innate source of power which is stronger or weaker as the case may be. I can see the appeal, since is makes the writing easier if characters can just make things happen without practice. As it happens, Harry has a very powerful magical core which could lead to him performing wandless magic before he can tie his shoes. Dumbledore decided to bind Harry's magical core, however, because he's evil.
3. The Anti-Horcrux. If Dumbledore had bothered to look it up, he'd have discovered that the procedure to harmlessly remove a horcrux from a living host is quite simple. He didn't bother, however, because he's evil.

These three represent the sliding scale of Evil Dumbledore. The first sets up a far more manipulative Dumbledore who is constantly abusing his power to keep Harry away from the wonderful and loving family desperately trying to adopt him and raise him to be the next William Gladstone by the time he's thirteen.

The second sets up a far more sinister Dumbledore who is actively harming Harry to maintain his own grip on power and the third depicts the quintessential Evil Dumbledore who is hellbent on an entirely needless blood sacrifice.

I find it very strange that these authors can't stick to the canonical list of grievances (leaving him on the doorstep in the middle of the night, keeping secrets and so on) and always include invented crimes which the reader is presumably meant to be incensed over. But what's the point? If you don't like Dumbledore then surely your characters can rake him over the coals for the things in the actual book that made you dislike him rather than for things you made him do.