"why the Islamic God,being THE MOST MERCIFUL get offensive and egoistic when someone does not Believe in his existence"
Allah does not get offended or egoistic but He has given us Revelation for our benefit. IF He tells you not to put your hand in fire, it will burn, and you put hand in fire and it burns, is Allah punishing you for disobedience, getting angry by not cooling the fire or is happy you did this?
Everything has a cause and effect, an action and reaction and when Allah is self-sufficient and has no need of anything, the last thing He would want or need is our 'servitude', lump it and be damned!
Your question is imagining a god in your image, not the one presented in the Qur'an. Not believing in Allah, or worshipping other gods, associating partners with Him, is all based on what is best for us, anything other than one God, one True Revelation will result in chaos, contradiction and absurdities, and we suffer as a result, not Him.
Allah is Merciful because He provided us with the means to survive, air, water, food, and we lose His 'Mercy' by polluting the earth and greed; did you make the earth or He and what thanks should you give Him? Deny His 'existence', deny future consequences, ungrateful lot are we!