the topic is "after a careful exploration of the development of macbeth's character in the play, it is undeniable that Macbeth is a tragic hero"
my two argument is 1- macbeth is a brave and loyal man at the beginning of the play 2-his ambition and manipulation of other people leads to his downfall
One might also be able to argue effectively that without the impetus of Lady Macbeth, Macbeth would have been able to overcome his desire for a more immediate acquisition of the throne as predicted by the hags. One might argue that the undue influence placed on him by Lady Macbeth led to the choices he made which led to his downfall.
While it is true that Macbeth is a brave and loyal soldier and subject at the beginning of the play, it certainly does not take much for his latent ambition to surface. When the witches predict he will one day be king, Macbeth is not moved. Mere hours later, when Duncan names his son as his heri, Macbeth says this is a stumbling block which he must "o'erleap." Perhaps he was not as loyal and content to be the king's cousin and soldier as he appears, even from the beginning.
I think you are on the right track. Macbeth does begin at a high and noble position in society and through his "vaulting ambition" he falls, literally and figuratively, to his death, bringing several with him.
Your arguments need to be examined. It is not really an argument that he is brave and loyal at the beginning of the play. You might begin this argument with "Because Macbeth is brave and loyal, he does not need to kill to attain power and position." This part will kill with his tragic flaw. For your second argument, consider why Macbeth falls. Ambition is one; so is overconfidence with the prophecies, and the deterioration of his ability to feel.