I kid you not, the root of all evil is language.
bear with me here, because there is more than just an ounce of "truth" in this and it's a really simple explanation. When humanity started to use language, a curious thing happened - intention and action were separated. If you do not have language, then you need to demonstrate your intentions by your actions. With the advent of language however, the ability to say one thing but do another, also called deception, was born. Language brought us the ability to lie and deceive and thus introduced the root of all evil to man.
Of course I should point out that at the same time it transformed our "good/bad-less acts of simply doing things" into a acts that can be classified on a gradient of "doing things that negatively impact people" to "doing things that positively impact people" (notice the word gradient, thinking in just good or evil would be increditbly stupid).
So language lets us lie, cheat, and deceive. A question that immediately arises is "Does that mean we do those things?" Well, if you were ever a kid (which unless you were tanked, you were) then you bet - we love doing those things, because they get us what we want. Even if later in life you found Jesus or Zen buddhism or some other belief that teaches you wrong from right, you probably still tell a lie and deceive someone from time to time. After all, giving up a "good thing" is hard to do.
But actually, that brings us to another terrible thing language brought us - mass religion and philosophy. To clarify, I do not mind religion or philosophy in the slightest. They offer excellent views on reality and if one or more of them works for you, this is good. But, I do mind people that believe their personal philosophy (be this religious or not) justifies hypocrisy (from the word hypocrisis, which comes from hupokrisis, which in turn comes from hupokrinesthai meaning to playact; a marginaly nuanced form of deception). Language has allowed us to transmit religion and philosophy beyond our own thoughts and convictions, through oral and written histories, and this very use of language has made both highly curious. Language changes, as does interpretation of language, so what is relayed or written down changes along with the language as times change, and can warp the original religion or philosophy.
Time makes fools of us all.
But of course only a fool would say "so abolish language" in response, as that would basically mean either becoming honest telepaths (the prospect of which for no good reason seems to scare people), or go back to a language-less society (which is reasonably impossible by now). Instead there should always be a question in the back of everyone's mind when confronted with any problem of any nature. "Is this a language inspired problem?"
Other than disputes that do not require language in any way (the "two dogs fighting over a bone" type of problem), all social problems are really just language problems. Either the language used was deceptive (ie, what was said was not actually what was meant), or it wasn't actually shared language (ie, the terms used did not describe the same thing to all parties).
Honestly, I love language, it's a fantastic thing to exploit, but you only really appreciate it when you realise it's the poster child of "things never to trust". The moment you assume that the things you are say both reflect what you mean as well as mean the same to you as they do to your listeners, you're bound to run into problems.
So to wrap it up, a definition; "Irony: Writing a brief on the fact that language cannot be trusted."