Why

i didnt too just until ch 42. loneal said he didnt wanna be a burden. he didnt wanna be a liability bc the man kept attacking him specifically making it hard for the prince to fight back so he decided to leave so the prince wouldnt be distracted

because like the the commenter above said, he's a liability towards the prince, he's basically the reason why the prince had to hold back and protect him because he's very weak, and the cloaked bastard is aiming for Loneal too, that's why he decided to jump off the cliff so the prince won't be held back (⌒▽⌒)

Adding too, he knew he'd fall asleep/collapse/fall unconscious after using such much power which would be an even bigger burden for Alec to protect him. So he removed himself from the equation. He knew the seal was below the cliff (little sketch shows its squeaking below them) and trusted it would pull his unconscious body from the water.

*so much

Ahhh makes sense