Slenderman is unarguably the most iconic creation of the early internet. It has led to games, stories and even books. But is his design evolutionarily possible? What evolutionary pressures would lead to the slender figure and abilities of the slenderman?
Humans are already basically Slenderman to other animals. In prehistoric times, we were persistence hunters - isolating a single animal and following it, slowly and persistently, until it could run no further, at which point we would jump out and kill it. Now what if there were a species that could out-persist us...
Slenderman's form is basically an exaggeration of the same features that distinguish us from other animals. Less musculature, but long limbs - perfect for long distance walking. It doesn't need claws or teeth, since its prey is already half-dead from exhaustion by the time it finally decides to attack. And of course, being very, very good at tracking, and very, very patient.
The creature could have the ability to produce infrasound. Certain pitches of infrasound are able to cause vague feelings of dread in humans and has even been thought to cause mild hallucinations. Many "haunted" houses have been found to have some object in them producing infrasound (like a vibrating pipe). Making the prey perpetually terrified is an excellent adaptation for a persistence hunter, as it will constantly be on the alert, fighting and running from things that aren't there, and generally wasting energy, making it tire out faster. The prey may also become paranoid, causing them to avoid other people, which is just what a persistence hunter wants. The hallucinations could account for the varying accounts of the creature's description. It probably doesn't really have tentacles, for example.
The creature must be stealthy, since if it is spotted before the prey is tired out, they might decide to attack it and could potentially fight it off. It must be able to stay perfectly still, and it makes sense that it would mainly inhabit forests, since it can be mistaken for a tree at a distance. However, humans (along with many other animals) are very good at spotting faces. For a predator that needs to remain unnoticed, lacking visible facial features is a boon. It seems to be solitary, so it doesn't need a face to express itself.
The messing with electronics thing is harder to explain, but there have been some recorded cases of humans who collect much more static electricity than normal and cause problems for electronic equipment near them. Perhaps this creature has similar properties. It's hard to say why it would have evolved with these qualities though.
Humans are not very good primary prey, as we are bony and lack meat, plus we can run far and are hard for a persistence hunter to catch. Slendy probably hunts other animals like rabbit and deer most of the time, just like we did. Perhaps the species likes hunting challenging creatures for sport, though. We do.