We have no idea who 'he'. We only assume it is a man because of his name and suit. Us, humans give names to something we have discovered, so Slender Man may not even be it's true name (if it has a concious). I can only assume what he is capable of and from my research he is dangerous. If it even exsists there would be those who would disect and experiment on it if given the chance to, it being in hidding almost makes it seem like it has a conciousness and is aware of what may happen if it shows it's self. Could that be why it kills? (If it even does kill) Is it so alone that it became a sadistic God that many of us who don't theorize, research, came upon by curiosity or love horror actulally worship this monster as a God. "We only know the consiquence of our action until we are in pain" -Unknown.
Off record I say this, "Those who are looking for this... 'thing' don't continue. Attempting at observational research is dangerous and can put you in serious harm." I am afriad no one will take no as an answer and will spend their whole lives trying to find something that may or may not exsist. Take care and don't think it won't find you.