The military parade and Kim's speech are indicative of the North Korean leader's grand nuclear ambitions and are a testament to his personal instability. Kim seeks to recklessly show the world what North Korea is capable of - especially to a new and more hawkish leader in the South.
North Korea has been pushed into a corner by the West. Pyongyang for years made a goodwill effort towards diplomacy with the West and denuclearization, but all it got in return was snubbed and strong-armed. Sanctions were not lightened, and agreements were continuously sabotaged.
Though it may seem that Kim is some sort of madman, he's simply using the only leverage he has in dealing with South Korea and the US. Kim's nuclear ambitions are purely for show to seek political gains and sanctions relief.