Logical, understandable reasons Mon-El would have lied
1) he’s new to earth. He’s ROYALTY. He didn’t know if he could trust kara and the DEO (they held him prisoner and Kara expressed hate for Daxam)
…do you know what happens to royalty in the hands of an enemy? Just watch Game of Thrones. He had to keep it secret so they wouldn’t keep him prisoner and try to use him for some hostage purposes.
2) by the time he knew he could trust them, he’d already lied to them. The damage was done.. AND HE HAD BEGUN TO FEEL ASHAMED of the man he used to be. And no one had responded to his beacon. He thought he was all alone. And if he admitted he was lying, he could possibly lose the only friends he had on this alien planet.
…the poor dude hated who he was before, he wanted to be Mon-El, hero of Earth. Partner to Kara. And in his head he was the “former” prince of Daxam. So the secret affected no one and held huge potential to cause him to lose everything…
TO SUM UP, he was *right* and *justified* to lie at the beginning. To keep lying after that wasn’t great but if you don’t understand his position you are doing your very best to be unempathetic and I hope you never make any mistakes in life because you wouldn’t like people being as hard on you as you are being of him.
It’s not like he, I dunno, killed Kara’s aunt… or assumed the identity of a DEO officer for years… or lied to everyone in his life for 15+ years about having super powers. Kara didn’t even tell Adam, the guy she was dating…