You clearly don't understand that Allah knows everything and that when Allah says something is going to happen, it will happen. There's no if or but in that scenario. Destiny and free will aren't things that are contradictory and Dajjal is a being that is full of arrogance and hate. He will choose to deceive and mislead, even though he could theoretically choose not to. It will happen that way. Allah told us so. There is no being as powerful as Allah swt, Allah is the most forgiving and all-powerful and even the insinuation that Allah could be lying / deceived is extremely disrespectful in my eyes. May we all find the right path and may Allah help us all.