Madhav very well knows Aru's nature... He knows that once Aru realises, at this point, that he is 'Ehsaas', all her 'love' will go out of the window as she always imagined a dashing, handsome man as Ehsaas, which Madhav is not ( Aru herself told him)...
He loves Aru so he doesn't want her to feel the pain which he is facing currently... He himself told Rahul that Aradhya will slowly stop searching Ehsaas in Rahul... They have even planned to trll te truth to Aradhya after marriage...I agree both guys are doing wrong, but blaming Madhav only is not justified...
Nobody is forcing Rahul to hide the truth... He is going on with this marriage just to make his Ma'am happy... and Madhav is hiding because he wants to make Aradhya happy, in his own way...
The decision by both guys whether us right or wrong, depends on the depth of Aradhya's love for Ehsaas...