Discord Wasn't all that mischevious, infact him and Celestia had a close bond.
Their childhood involved playing games with each other and learning. Whilst in the teenage years they began a closer relationship. But when Celesita had told her parents her undying love for Discord they disaproved and kept her away and forced them apart. Out of out rage Discord became Chaotic for his actions. Celestia couldn't understand why the alicorn she loved turned on the kingdoms back. The war Began.